diff --git a/apps/solver/equation_store.cpp b/apps/solver/equation_store.cpp
index 109ca304e..297d0530a 100644
--- a/apps/solver/equation_store.cpp
+++ b/apps/solver/equation_store.cpp
@@ -150,7 +150,7 @@ EquationStore::Error EquationStore::exactSolve(Poincare::Context * context) {
   } else {
     /* 2- Polynomial & Monovariable? */
     assert(numberOfVariables == 1 && numberOfDefinedModels() == 1);
-    char x = m_variables[0];
+    const char x[] = {m_variables[0], 0};
     Expression polynomialCoefficients[Expression::k_maxNumberOfPolynomialCoefficients];
     int degree = definedModelAtIndex(0)->standardForm(context).getPolynomialReducedCoefficients(x, polynomialCoefficients, *context, preferences->angleUnit());
     if (degree == 2) {
diff --git a/poincare/include/poincare/addition.h b/poincare/include/poincare/addition.h
index e1e40f5af..b05030626 100644
--- a/poincare/include/poincare/addition.h
+++ b/poincare/include/poincare/addition.h
@@ -26,8 +26,8 @@ public:
 
   // Properties
   Type type() const override { return Type::Addition; }
-  int polynomialDegree(Context & context, char symbolName) const override;
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const override;
 
   // Evaluation
   template<typename T> static Complex<T> compute(const std::complex<T> c, const std::complex<T> d) { return Complex<T>(c+d); }
@@ -81,7 +81,7 @@ public:
   // Expression
   Expression shallowReduce(Context & context, Preferences::AngleUnit angleUnit);
   Expression shallowBeautify(Context & context, Preferences::AngleUnit angleUnit);
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const;
 private:
   static const Number NumeralFactor(const Expression & e);
   static inline int NumberOfNonNumeralFactors(const Expression & e);
diff --git a/poincare/include/poincare/confidence_interval.h b/poincare/include/poincare/confidence_interval.h
index 73190ec38..93e8a9e83 100644
--- a/poincare/include/poincare/confidence_interval.h
+++ b/poincare/include/poincare/confidence_interval.h
@@ -21,7 +21,7 @@ public:
 
   // Properties
   Type type() const override { return Type::ConfidenceInterval; }
-  int polynomialDegree(Context & context, char symbolName) const override { return -1; }
+  int polynomialDegree(Context & context, const char * symbolName) const override { return -1; }
 private:
   // Layout
   Layout createLayout(Preferences::PrintFloatMode floatDisplayMode, int numberOfSignificantDigits) const override;
diff --git a/poincare/include/poincare/derivative.h b/poincare/include/poincare/derivative.h
index fce78d995..b899a5541 100644
--- a/poincare/include/poincare/derivative.h
+++ b/poincare/include/poincare/derivative.h
@@ -22,7 +22,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Derivative; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
 
 private:
   // Layout
diff --git a/poincare/include/poincare/division.h b/poincare/include/poincare/division.h
index 5162499aa..297ea35ed 100644
--- a/poincare/include/poincare/division.h
+++ b/poincare/include/poincare/division.h
@@ -25,7 +25,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Division; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
 
   // Approximation
   virtual Evaluation<float> approximate(SinglePrecision p, Context& context, Preferences::AngleUnit angleUnit) const override {
diff --git a/poincare/include/poincare/equal.h b/poincare/include/poincare/equal.h
index 6883aee27..b1d5946dd 100644
--- a/poincare/include/poincare/equal.h
+++ b/poincare/include/poincare/equal.h
@@ -19,7 +19,7 @@ public:
 
   // ExpressionNode
   Type type() const override { return Type::Equal; }
-  int polynomialDegree(Context & context, char symbolName) const override { return -1; }
+  int polynomialDegree(Context & context, const char * symbolName) const override { return -1; }
 private:
   // Simplification
   Expression shallowReduce(Context & context, Preferences::AngleUnit angleUnit) override;
diff --git a/poincare/include/poincare/expression.h b/poincare/include/poincare/expression.h
index 39ab9b78f..581244817 100644
--- a/poincare/include/poincare/expression.h
+++ b/poincare/include/poincare/expression.h
@@ -123,7 +123,7 @@ public:
   /* polynomialDegree returns:
    * - (-1) if the expression is not a polynome
    * - the degree of the polynome otherwise */
-  int polynomialDegree(Context & context, char symbolName) const { return this->node()->polynomialDegree(context, symbolName); }
+  int polynomialDegree(Context & context, const char * symbolName) const { return this->node()->polynomialDegree(context, symbolName); }
   /* getVariables fills the table variables with the variable present in the
    * expression and returns the number of entries in filled in variables.
    * For instance getVariables('x+y+2*w/cos(4)') would result in
@@ -214,7 +214,7 @@ protected:
   void removeChildrenInPlace(int currentNumberOfChildren) = delete;
 
   /* Properties */
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const { return node()->getPolynomialCoefficients(context, symbolName, coefficients); }
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const { return node()->getPolynomialCoefficients(context, symbolName, coefficients); }
 
   /* Simplification */
   Expression denominator(Context & context, Preferences::AngleUnit angleUnit) const { return node()->denominator(context, angleUnit); }
@@ -232,7 +232,7 @@ private:
 
   /* Properties */
   Expression defaultReplaceSymbolWithExpression(const char * symbol, Expression expression);
-  int defaultGetPolynomialCoefficients(Context & context, char symbol, Expression expression[]) const;
+  int defaultGetPolynomialCoefficients(Context & context, const char * symbol, Expression expression[]) const;
 
   /* Expression roots/extrema solver*/
   constexpr static double k_solverPrecision = 1.0E-5;
diff --git a/poincare/include/poincare/expression_node.h b/poincare/include/poincare/expression_node.h
index 03e84fc95..0a625b1cc 100644
--- a/poincare/include/poincare/expression_node.h
+++ b/poincare/include/poincare/expression_node.h
@@ -101,8 +101,8 @@ public:
   virtual bool isNumber() const { return false; }
   /*!*/ virtual Expression replaceSymbolWithExpression(const char * symbol, Expression & expression);
   /*!*/ virtual Expression setSign(Sign s, Context & context, Preferences::AngleUnit angleUnit);
-  virtual int polynomialDegree(Context & context, char symbolName) const;
-  /*!*/ virtual int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const;
+  virtual int polynomialDegree(Context & context, const char * symbolName) const;
+  /*!*/ virtual int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const;
   typedef bool (*isVariableTest)(char c);
   virtual int getVariables(Context & context, isVariableTest isVariable, char * variables) const;
   virtual float characteristicXRange(Context & context, Preferences::AngleUnit angleUnit) const;
diff --git a/poincare/include/poincare/function.h b/poincare/include/poincare/function.h
index a67d7340d..d007a6cfe 100644
--- a/poincare/include/poincare/function.h
+++ b/poincare/include/poincare/function.h
@@ -18,8 +18,8 @@ public:
 
   // Properties
   Type type() const override { return Type::Function; }
-  int polynomialDegree(Context & context, char symbolName) const override;
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const override;
   int getVariables(Context & context, isVariableTest isVariable, char * variables) const override;
   float characteristicXRange(Context & context, Preferences::AngleUnit angleUnit) const override;
 
diff --git a/poincare/include/poincare/integral.h b/poincare/include/poincare/integral.h
index df3431225..db2a3c414 100644
--- a/poincare/include/poincare/integral.h
+++ b/poincare/include/poincare/integral.h
@@ -19,7 +19,7 @@ public:
 
   // ExpressionNode
   Type type() const override { return Type::Integral; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
 private:
   // Layout
   Layout createLayout(Preferences::PrintFloatMode floatDisplayMode, int numberOfSignificantDigits) const override;
diff --git a/poincare/include/poincare/matrix.h b/poincare/include/poincare/matrix.h
index 6b1930000..1742543ec 100644
--- a/poincare/include/poincare/matrix.h
+++ b/poincare/include/poincare/matrix.h
@@ -36,7 +36,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Matrix; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
 
   // Approximation
   Evaluation<float> approximate(SinglePrecision p, Context& context, Preferences::AngleUnit angleUnit) const override {
diff --git a/poincare/include/poincare/multiplication.h b/poincare/include/poincare/multiplication.h
index 1616d1acc..e66937972 100644
--- a/poincare/include/poincare/multiplication.h
+++ b/poincare/include/poincare/multiplication.h
@@ -22,8 +22,8 @@ public:
   // Properties
   Type type() const override { return Type::Multiplication; }
   Sign sign() const override;
-  int polynomialDegree(Context & context, char symbolName) const override;
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const override;
 
   // Approximation
   template<typename T> static Complex<T> compute(const std::complex<T> c, const std::complex<T> d) { return Complex<T>(c*d); }
@@ -83,7 +83,7 @@ public:
   Expression setSign(ExpressionNode::Sign s, Context & context, Preferences::AngleUnit angleUnit);
   Expression shallowReduce(Context & context, Preferences::AngleUnit angleUnit);
   Expression shallowBeautify(Context & context, Preferences::AngleUnit angleUnit);
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const;
   Expression denominator(Context & context, Preferences::AngleUnit angleUnit) const;
 private:
   // Simplification
diff --git a/poincare/include/poincare/opposite.h b/poincare/include/poincare/opposite.h
index 60a8a59a0..2fb981a3a 100644
--- a/poincare/include/poincare/opposite.h
+++ b/poincare/include/poincare/opposite.h
@@ -24,7 +24,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Opposite; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
   Sign sign() const override;
 
   // Approximation
diff --git a/poincare/include/poincare/parenthesis.h b/poincare/include/poincare/parenthesis.h
index 54a93734b..c5bbf9068 100644
--- a/poincare/include/poincare/parenthesis.h
+++ b/poincare/include/poincare/parenthesis.h
@@ -20,7 +20,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Parenthesis; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
 
   // Layout
   Layout createLayout(Preferences::PrintFloatMode floatDisplayMode, int numberOfSignificantDigits) const override;
diff --git a/poincare/include/poincare/power.h b/poincare/include/poincare/power.h
index 4e130b0df..d53a85eb8 100644
--- a/poincare/include/poincare/power.h
+++ b/poincare/include/poincare/power.h
@@ -27,8 +27,8 @@ public:
   Sign sign() const override;
   Expression setSign(Sign s, Context & context, Preferences::AngleUnit angleUnit) override;
 
-  int polynomialDegree(Context & context, char symbolName) const override;
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const override;
 
   template<typename T> static Complex<T> compute(const std::complex<T> c, const std::complex<T> d);
 
@@ -70,7 +70,7 @@ public:
   Power(Expression base, Expression exponent);
   Power(const PowerNode * n) : Expression(n) {}
   Expression setSign(ExpressionNode::Sign s, Context & context, Preferences::AngleUnit angleUnit);
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const;
   Expression shallowReduce(Context & context, Preferences::AngleUnit angleUnit);
   Expression shallowBeautify(Context & context, Preferences::AngleUnit angleUnit);
 
diff --git a/poincare/include/poincare/prediction_interval.h b/poincare/include/poincare/prediction_interval.h
index f8931af6c..9dd9b52b9 100644
--- a/poincare/include/poincare/prediction_interval.h
+++ b/poincare/include/poincare/prediction_interval.h
@@ -23,7 +23,7 @@ public:
 
   // Properties
   Type type() const override { return Type::PredictionInterval; }
-  int polynomialDegree(Context & context, char symbolName) const override { return -1; }
+  int polynomialDegree(Context & context, const char * symbolName) const override { return -1; }
 private:
   // Layout
   Layout createLayout(Preferences::PrintFloatMode floatDisplayMode, int numberOfSignificantDigits) const override;
diff --git a/poincare/include/poincare/store.h b/poincare/include/poincare/store.h
index 4fcce4c7d..7fefa97b2 100644
--- a/poincare/include/poincare/store.h
+++ b/poincare/include/poincare/store.h
@@ -22,7 +22,7 @@ public:
 
   // ExpressionNode
   Type type() const override { return Type::Store; }
-  int polynomialDegree(Context & context, char symbolName) const override { return -1; }
+  int polynomialDegree(Context & context, const char * symbolName) const override { return -1; }
 
 private:
   // Simplification
diff --git a/poincare/include/poincare/subtraction.h b/poincare/include/poincare/subtraction.h
index f7a50553c..83d6910e3 100644
--- a/poincare/include/poincare/subtraction.h
+++ b/poincare/include/poincare/subtraction.h
@@ -23,7 +23,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Subtraction; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
 
   // Approximation
   template<typename T> static Complex<T> compute(const std::complex<T> c, const std::complex<T> d) { return Complex<T>(c - d); }
diff --git a/poincare/include/poincare/symbol.h b/poincare/include/poincare/symbol.h
index 79acbbc00..3d0ef4822 100644
--- a/poincare/include/poincare/symbol.h
+++ b/poincare/include/poincare/symbol.h
@@ -32,8 +32,8 @@ public:
   Type type() const override { return Type::Symbol; }
   Sign sign() const override;
   Expression replaceSymbolWithExpression(const char * symbol, Expression & expression) override;
-  int polynomialDegree(Context & context, char symbolName) const override;
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const override;
   int getVariables(Context & context, isVariableTest isVariable, char * variables) const override;
   float characteristicXRange(Context & context, Preferences::AngleUnit angleUnit) const override;
 
@@ -81,7 +81,7 @@ public:
   // Expression
   Expression shallowReduce(Context & context, Preferences::AngleUnit angleUnit);
   Expression replaceSymbolWithExpression(const char * symbol, Expression & expression);
-  int getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const;
+  int getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const;
 
   // Symbol
   const char * name() const { return node()->name(); }
diff --git a/poincare/include/poincare/undefined.h b/poincare/include/poincare/undefined.h
index 14f0626cf..6ebc2b47a 100644
--- a/poincare/include/poincare/undefined.h
+++ b/poincare/include/poincare/undefined.h
@@ -18,7 +18,7 @@ public:
 
   // Properties
   Type type() const override { return Type::Undefined; }
-  int polynomialDegree(Context & context, char symbolName) const override;
+  int polynomialDegree(Context & context, const char * symbolName) const override;
   Expression setSign(Sign s, Context & context, Preferences::AngleUnit angleUnit) override;
 
   // Approximation
diff --git a/poincare/src/addition.cpp b/poincare/src/addition.cpp
index ccf4739c0..9e5f09c3f 100644
--- a/poincare/src/addition.cpp
+++ b/poincare/src/addition.cpp
@@ -9,7 +9,7 @@
 
 namespace Poincare {
 
-int AdditionNode::polynomialDegree(Context & context, char symbolName) const {
+int AdditionNode::polynomialDegree(Context & context, const char * symbolName) const {
   int degree = 0;
   for (ExpressionNode * e : children()) {
     int d = e->polynomialDegree(context, symbolName);
@@ -21,7 +21,7 @@ int AdditionNode::polynomialDegree(Context & context, char symbolName) const {
   return degree;
 }
 
-int AdditionNode::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int AdditionNode::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   return Addition(this).getPolynomialCoefficients(context, symbolName, coefficients);
 }
 
@@ -60,7 +60,7 @@ const Number Addition::NumeralFactor(const Expression & e) {
   return Rational(1);
 }
 
-int Addition::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int Addition::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   int deg = polynomialDegree(context, symbolName);
   if (deg < 0 || deg > Expression::k_maxPolynomialDegree) {
     return -1;
diff --git a/poincare/src/derivative.cpp b/poincare/src/derivative.cpp
index 5788ce054..c37b93f01 100644
--- a/poincare/src/derivative.cpp
+++ b/poincare/src/derivative.cpp
@@ -8,7 +8,7 @@
 
 namespace Poincare {
 
-int DerivativeNode::polynomialDegree(Context & context, char symbolName) const {
+int DerivativeNode::polynomialDegree(Context & context, const char * symbolName) const {
   if (childAtIndex(0)->polynomialDegree(context, symbolName) == 0
       && childAtIndex(1)->polynomialDegree(context, symbolName) == 0)
   {
diff --git a/poincare/src/division.cpp b/poincare/src/division.cpp
index 1343afd30..42d84b90b 100644
--- a/poincare/src/division.cpp
+++ b/poincare/src/division.cpp
@@ -11,7 +11,7 @@
 
 namespace Poincare {
 
-int DivisionNode::polynomialDegree(Context & context, char symbolName) const {
+int DivisionNode::polynomialDegree(Context & context, const char * symbolName) const {
   if (childAtIndex(1)->polynomialDegree(context, symbolName) != 0) {
     return -1;
   }
diff --git a/poincare/src/expression.cpp b/poincare/src/expression.cpp
index b11c1c6bb..d6c98b30d 100644
--- a/poincare/src/expression.cpp
+++ b/poincare/src/expression.cpp
@@ -105,7 +105,8 @@ bool Expression::getLinearCoefficients(char * variables, Expression coefficients
   assert(!recursivelyMatches(IsMatrix, context));
   char * x = variables;
   while (*x != 0) {
-    int degree = polynomialDegree(context, *x);
+    const char symbol[] = {*x, 0};
+    int degree = polynomialDegree(context, symbol);
     if (degree > 1 || degree < 0) {
       return false;
     }
@@ -116,7 +117,8 @@ bool Expression::getLinearCoefficients(char * variables, Expression coefficients
   int index = 0;
   Expression polynomialCoefficients[k_maxNumberOfPolynomialCoefficients];
   while (*x != 0) {
-    int degree = equation.getPolynomialReducedCoefficients(*x, polynomialCoefficients, context, angleUnit);
+    const char symbol[] = {*x, 0};
+    int degree = equation.getPolynomialReducedCoefficients(symbol, polynomialCoefficients, context, angleUnit);
     switch (degree) {
       case 0:
         coefficients[index] = Rational(0);
@@ -189,7 +191,7 @@ Expression Expression::defaultReplaceSymbolWithExpression(const char * symbol, E
   return *this;
 }
 
-int Expression::defaultGetPolynomialCoefficients(Context & context, char symbol, Expression coefficients[]) const {
+int Expression::defaultGetPolynomialCoefficients(Context & context, const char * symbol, Expression coefficients[]) const {
   int deg = polynomialDegree(context, symbol);
   if (deg == 0) {
     coefficients[0] = clone();
@@ -198,7 +200,7 @@ int Expression::defaultGetPolynomialCoefficients(Context & context, char symbol,
   return -1;
 }
 
-int Expression::getPolynomialReducedCoefficients(char symbolName, Expression coefficients[], Context & context, Preferences::AngleUnit angleUnit) const {
+int Expression::getPolynomialReducedCoefficients(const char * symbolName, Expression coefficients[], Context & context, Preferences::AngleUnit angleUnit) const {
   int degree = getPolynomialCoefficients(context, symbolName, coefficients);
   for (int i = 0; i <= degree; i++) {
     coefficients[i] = coefficients[i].deepReduce(context, angleUnit);
diff --git a/poincare/src/expression_node.cpp b/poincare/src/expression_node.cpp
index 58faac70c..439d7b9a1 100644
--- a/poincare/src/expression_node.cpp
+++ b/poincare/src/expression_node.cpp
@@ -13,7 +13,7 @@ Expression ExpressionNode::setSign(Sign s, Context & context, Preferences::Angle
   return Expression();
 }
 
-int ExpressionNode::polynomialDegree(Context & context, char symbolName) const {
+int ExpressionNode::polynomialDegree(Context & context, const char * symbolName) const {
   for (ExpressionNode * c : children()) {
     if (c->polynomialDegree(context, symbolName) != 0) {
       return -1;
@@ -22,7 +22,7 @@ int ExpressionNode::polynomialDegree(Context & context, char symbolName) const {
   return 0;
 }
 
-int ExpressionNode::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int ExpressionNode::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   return Expression(this).defaultGetPolynomialCoefficients(context, symbolName, coefficients);
 }
 
diff --git a/poincare/src/function.cpp b/poincare/src/function.cpp
index 4c12881c5..333f4b60d 100644
--- a/poincare/src/function.cpp
+++ b/poincare/src/function.cpp
@@ -7,12 +7,12 @@
 
 namespace Poincare {
 
-int FunctionNode::polynomialDegree(Context & context, char symbolName) const {
+int FunctionNode::polynomialDegree(Context & context, const char * symbolName) const {
   Expression e = context.expressionForSymbol(Function(this));
   return e.polynomialDegree(context, symbolName);
 }
 
-int FunctionNode::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int FunctionNode::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   Expression e = context.expressionForSymbol(Function(this));
   return e.getPolynomialCoefficients(context, symbolName, coefficients);
 }
diff --git a/poincare/src/integral.cpp b/poincare/src/integral.cpp
index bf826889e..58373aa22 100644
--- a/poincare/src/integral.cpp
+++ b/poincare/src/integral.cpp
@@ -9,7 +9,7 @@
 
 namespace Poincare {
 
-int IntegralNode::polynomialDegree(Context & context, char symbolName) const {
+int IntegralNode::polynomialDegree(Context & context, const char * symbolName) const {
   if (childAtIndex(0)->polynomialDegree(context, symbolName) == 0
       && childAtIndex(1)->polynomialDegree(context, symbolName) == 0
       && childAtIndex(2)->polynomialDegree(context, symbolName) == 0)
diff --git a/poincare/src/matrix.cpp b/poincare/src/matrix.cpp
index 64786a4ad..6d291e102 100644
--- a/poincare/src/matrix.cpp
+++ b/poincare/src/matrix.cpp
@@ -17,7 +17,7 @@ void MatrixNode::didAddChildAtIndex(int newNumberOfChildren) {
   setNumberOfColumns(newNumberOfChildren);
 }
 
-int MatrixNode::polynomialDegree(Context & context, char symbolName) const {
+int MatrixNode::polynomialDegree(Context & context, const char * symbolName) const {
   return -1;
 }
 
diff --git a/poincare/src/multiplication.cpp b/poincare/src/multiplication.cpp
index 982e5c06c..20f22838d 100644
--- a/poincare/src/multiplication.cpp
+++ b/poincare/src/multiplication.cpp
@@ -28,7 +28,7 @@ ExpressionNode::Sign MultiplicationNode::sign() const {
   return (Sign)sign;
 }
 
-int MultiplicationNode::polynomialDegree(Context & context, char symbolName) const {
+int MultiplicationNode::polynomialDegree(Context & context, const char * symbolName) const {
   int degree = 0;
   for (ExpressionNode * c : children()) {
     int d = c->polynomialDegree(context, symbolName);
@@ -40,7 +40,7 @@ int MultiplicationNode::polynomialDegree(Context & context, char symbolName) con
   return degree;
 }
 
-int MultiplicationNode::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int MultiplicationNode::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   return Multiplication(this).getPolynomialCoefficients(context, symbolName, coefficients);
 }
 
@@ -195,7 +195,7 @@ Expression Multiplication::shallowBeautify(Context & context, Preferences::Angle
   return thisExp;
 }
 
-int Multiplication::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int Multiplication::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   int deg = polynomialDegree(context, symbolName);
   if (deg < 0 || deg > Expression::k_maxPolynomialDegree) {
     return -1;
diff --git a/poincare/src/opposite.cpp b/poincare/src/opposite.cpp
index f1b1f05b4..ac829e0db 100644
--- a/poincare/src/opposite.cpp
+++ b/poincare/src/opposite.cpp
@@ -13,7 +13,7 @@ extern "C" {
 
 namespace Poincare {
 
-int OppositeNode::polynomialDegree(Context & context, char symbolName) const {
+int OppositeNode::polynomialDegree(Context & context, const char * symbolName) const {
   return childAtIndex(0)->polynomialDegree(context, symbolName);
 }
 
diff --git a/poincare/src/parenthesis.cpp b/poincare/src/parenthesis.cpp
index 893982c64..500003b81 100644
--- a/poincare/src/parenthesis.cpp
+++ b/poincare/src/parenthesis.cpp
@@ -3,7 +3,7 @@
 
 namespace Poincare {
 
-int ParenthesisNode::polynomialDegree(Context & context, char symbolName) const {
+int ParenthesisNode::polynomialDegree(Context & context, const char * symbolName) const {
   return childAtIndex(0)->polynomialDegree(context, symbolName);
 }
 
diff --git a/poincare/src/power.cpp b/poincare/src/power.cpp
index 613bb46e9..d947272da 100644
--- a/poincare/src/power.cpp
+++ b/poincare/src/power.cpp
@@ -48,7 +48,7 @@ Expression PowerNode::setSign(Sign s, Context & context, Preferences::AngleUnit
   return Power(this).setSign(s, context, angleUnit);
 }
 
-int PowerNode::polynomialDegree(Context & context, char symbolName) const {
+int PowerNode::polynomialDegree(Context & context, const char * symbolName) const {
   int deg = ExpressionNode::polynomialDegree(context, symbolName);
   if (deg == 0) {
     return deg;
@@ -72,7 +72,7 @@ int PowerNode::polynomialDegree(Context & context, char symbolName) const {
   return -1;
 }
 
-int PowerNode::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int PowerNode::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   return Power(this).getPolynomialCoefficients(context, symbolName, coefficients);
 }
 
@@ -206,7 +206,7 @@ Expression Power::setSign(ExpressionNode::Sign s, Context & context, Preferences
   return result;
 }
 
-int Power::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int Power::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   int deg = polynomialDegree(context, symbolName);
   if (deg <= 0) {
     return Expression::defaultGetPolynomialCoefficients(context, symbolName, coefficients);
diff --git a/poincare/src/subtraction.cpp b/poincare/src/subtraction.cpp
index f60564fc5..d48a13392 100644
--- a/poincare/src/subtraction.cpp
+++ b/poincare/src/subtraction.cpp
@@ -8,7 +8,7 @@
 
 namespace Poincare {
 
-int SubtractionNode::polynomialDegree(Context & context, char symbolName) const {
+int SubtractionNode::polynomialDegree(Context & context, const char * symbolName) const {
   int degree = 0;
   for (ExpressionNode * e : children()) {
     int d = e->polynomialDegree(context, symbolName);
diff --git a/poincare/src/symbol.cpp b/poincare/src/symbol.cpp
index 7153b7c5d..ecceca507 100644
--- a/poincare/src/symbol.cpp
+++ b/poincare/src/symbol.cpp
@@ -34,14 +34,14 @@ Expression SymbolNode::replaceSymbolWithExpression(const char * symbol, Expressi
   return Symbol(this).replaceSymbolWithExpression(symbol, expression);
 }
 
-int SymbolNode::polynomialDegree(Context & context, char symbolName) const {
-  if (m_name == symbolName) {
+int SymbolNode::polynomialDegree(Context & context, const char * symbolName) const {
+  if (strcmp(m_name,symbolName) == 0) {
     return 1;
   }
   return 0;
 }
 
-int SymbolNode::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int SymbolNode::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   return Symbol(this).getPolynomialCoefficients(context, symbolName, coefficients);
 }
 
@@ -196,7 +196,7 @@ Expression Symbol::replaceSymbolWithExpression(const char * symbol, Expression &
   return *this;
 }
 
-int Symbol::getPolynomialCoefficients(Context & context, char symbolName, Expression coefficients[]) const {
+int Symbol::getPolynomialCoefficients(Context & context, const char * symbolName, Expression coefficients[]) const {
   if (name() == symbolName) {
     coefficients[0] = Rational(0);
     coefficients[1] = Rational(1);
diff --git a/poincare/src/trigonometry.cpp b/poincare/src/trigonometry.cpp
index 3af27172a..814e352f1 100644
--- a/poincare/src/trigonometry.cpp
+++ b/poincare/src/trigonometry.cpp
@@ -17,7 +17,8 @@ namespace Poincare {
 
 float Trigonometry::characteristicXRange(const Expression & e, Context & context, Preferences::AngleUnit angleUnit) {
   assert(e.numberOfChildren() == 1);
-  int d = e.childAtIndex(0).polynomialDegree(context, SpecialSymbols::UnknownX);
+  const char x[] = {SpecialSymbols::UnknownX, 0};
+  int d = e.childAtIndex(0).polynomialDegree(context, x);
   if (d < 0 || d > 1) {
     // child(0) is not linear so we cannot easily find an interesting range
     return e.childAtIndex(0).characteristicXRange(context, angleUnit);
diff --git a/poincare/src/undefined.cpp b/poincare/src/undefined.cpp
index 6e2fd0ad3..9723b2c2f 100644
--- a/poincare/src/undefined.cpp
+++ b/poincare/src/undefined.cpp
@@ -9,7 +9,7 @@ extern "C" {
 
 namespace Poincare {
 
-int UndefinedNode::polynomialDegree(Context & context, char symbolName) const {
+int UndefinedNode::polynomialDegree(Context & context, const char * symbolName) const {
   return -1;
 }
 
diff --git a/poincare/test/helper.cpp b/poincare/test/helper.cpp
index f5f2ce5eb..b4d7e0ffd 100644
--- a/poincare/test/helper.cpp
+++ b/poincare/test/helper.cpp
@@ -79,7 +79,7 @@ void assert_parsed_expression_is(const char * expression, Poincare::Expression r
   quiz_assert(e.isIdenticalTo(r));
 }
 
-void assert_parsed_expression_polynomial_degree(const char * expression, int degree, char symbolName) {
+void assert_parsed_expression_polynomial_degree(const char * expression, int degree, const char * symbolName) {
   GlobalContext globalContext;
   Expression e = parse_expression(expression);
   e = e.simplify(globalContext, Radian);
diff --git a/poincare/test/helper.h b/poincare/test/helper.h
index 185e845c5..8a3be178a 100644
--- a/poincare/test/helper.h
+++ b/poincare/test/helper.h
@@ -19,7 +19,7 @@ void translate_in_ASCII_chars(char * expression);
 Poincare::Expression parse_expression(const char * expression);
 void assert_parsed_expression_type(const char * expression, Poincare::ExpressionNode::Type type);
 void assert_parsed_expression_is(const char * expression, Poincare::Expression r);
-void assert_parsed_expression_polynomial_degree(const char * expression, int degree, char symbolName = 'x');
+void assert_parsed_expression_polynomial_degree(const char * expression, int degree, const char * symbolName = "x");
 
 template<typename T>
 void assert_parsed_expression_evaluates_to(const char * expression, const char * approximation, Poincare::Preferences::AngleUnit angleUnit = Degree, Poincare::Preferences::ComplexFormat complexFormat = Cartesian, int numberOfSignificantDigits = -1);
diff --git a/poincare/test/properties.cpp b/poincare/test/properties.cpp
index 6bf4d6164..5f699b4fe 100644
--- a/poincare/test/properties.cpp
+++ b/poincare/test/properties.cpp
@@ -104,7 +104,7 @@ QUIZ_CASE(poincare_get_variables) {
   assert_parsed_expression_has_variables("x^2+2*y+k!*A+w", "xykw");
 }
 
-void assert_parsed_expression_has_polynomial_coefficient(const char * expression, char symbolName, const char ** coefficients, Preferences::AngleUnit angleUnit = Preferences::AngleUnit::Degree) {
+void assert_parsed_expression_has_polynomial_coefficient(const char * expression, const char * symbolName, const char ** coefficients, Preferences::AngleUnit angleUnit = Preferences::AngleUnit::Degree) {
   GlobalContext globalContext;
   Expression e = parse_expression(expression);
   quiz_assert(!e.isUninitialized());
@@ -123,12 +123,12 @@ void assert_parsed_expression_has_polynomial_coefficient(const char * expression
 
 QUIZ_CASE(poincare_get_polynomial_coefficients) {
   const char * coefficient0[] = {"2", "1", "1", 0};
-  assert_parsed_expression_has_polynomial_coefficient("x^2+x+2", 'x', coefficient0);
+  assert_parsed_expression_has_polynomial_coefficient("x^2+x+2", "x", coefficient0);
   const char * coefficient1[] = {"12+(-6)*P", "12", "3", 0}; //3*x^2+12*x-6*π+12
-  assert_parsed_expression_has_polynomial_coefficient("3*(x+2)^2-6*P", 'x', coefficient1);
+  assert_parsed_expression_has_polynomial_coefficient("3*(x+2)^2-6*P", "x", coefficient1);
   // TODO: decomment when enable 3-degree polynomes
   //const char * coefficient2[] = {"2+32*x", "2", "6", "2", 0}; //2*n^3+6*n^2+2*n+2+32*x
-  //assert_parsed_expression_has_polynomial_coefficient("2*(n+1)^3-4n+32*x", 'n', coefficient2);
+  //assert_parsed_expression_has_polynomial_coefficient("2*(n+1)^3-4n+32*x", "n", coefficient2);
   const char * coefficient3[] = {"1", "-P", "1", 0}; //x^2-Pi*x+1
-  assert_parsed_expression_has_polynomial_coefficient("x^2-P*x+1", 'x', coefficient3);
+  assert_parsed_expression_has_polynomial_coefficient("x^2-P*x+1", "x", coefficient3);
 }