diff --git a/ext/symengine/ruby_function.c b/ext/symengine/ruby_function.c
index e8be33a..017a8b7 100644
--- a/ext/symengine/ruby_function.c
+++ b/ext/symengine/ruby_function.c
@@ -1,5 +1,7 @@
 #include "ruby_function.h"
 
+typedef struct CVecBasic CVecBasic;
+
 #define IMPLEMENT_ONE_ARG_FUNC(func) \
 VALUE cfunction_ ## func(VALUE self, VALUE operand1) { \
     return function_onearg(basic_ ## func, operand1); \
@@ -37,3 +39,36 @@ IMPLEMENT_ONE_ARG_FUNC(gamma);
 
 #undef IMPLEMENT_ONE_ARG_FUNC
 
+VALUE cfunction_functionsymbol_init(VALUE self, VALUE args)
+{
+    int argc = RARRAY_LEN(args);
+    if (argc == 0) {
+        rb_raise(rb_eTypeError, "Arguments Expected");
+    }
+
+    VALUE first = rb_ary_shift(args);
+    if (TYPE(first) != T_STRING) {
+        rb_raise(rb_eTypeError, "String expected as first argument");
+    }
+    char *name = StringValueCStr(first);
+    
+    CVecBasic *cargs = vecbasic_new();
+
+    basic x;
+    basic_new_stack(x);
+    int i;
+    for (i = 0; i < argc-1; i++) {
+        sympify(rb_ary_shift(args), x);
+        vecbasic_push_back(cargs, x);
+    }
+    
+    basic_struct *this;
+    Data_Get_Struct(self, basic_struct, this);
+    function_symbol_set(this, name, cargs);
+    
+    vecbasic_free(cargs);
+    basic_free_stack(x);
+
+    return self;
+}
+
diff --git a/ext/symengine/ruby_function.h b/ext/symengine/ruby_function.h
index e55b63b..b271353 100644
--- a/ext/symengine/ruby_function.h
+++ b/ext/symengine/ruby_function.h
@@ -37,4 +37,6 @@ VALUE cfunction_dirichlet_eta(VALUE self, VALUE operand1);
 VALUE cfunction_zeta(VALUE self, VALUE operand1);
 VALUE cfunction_gamma(VALUE self, VALUE operand1);
 
+VALUE cfunction_functionsymbol_init(VALUE self, VALUE args);
+
 #endif //RUBY_FUNCTION_H_
diff --git a/ext/symengine/symengine.c b/ext/symengine/symengine.c
index 3be164b..f86969e 100644
--- a/ext/symengine/symengine.c
+++ b/ext/symengine/symengine.c
@@ -110,6 +110,9 @@ void Init_symengine() {
     rb_define_const(m_symengine, "I", cconstant_i());
     rb_define_const(m_symengine, "HAVE_MPFR", cconstant_have_mpfr());
     rb_define_const(m_symengine, "HAVE_MPC", cconstant_have_mpc());
+    
+    //Subs class
+    c_subs = rb_define_class_under(m_symengine, "Subs", c_basic);
 
     //Add class
     c_add = rb_define_class_under(m_symengine, "Add", c_basic);
@@ -130,10 +133,12 @@ void Init_symengine() {
     c_dirichlet_eta = rb_define_class_under(m_symengine, "Dirichlet_eta", c_function);
     c_zeta = rb_define_class_under(m_symengine, "Zeta", c_function);
     c_gamma = rb_define_class_under(m_symengine, "Gamma", c_function);
-
-    //Abs Class
     c_abs = rb_define_class_under(m_symengine, "Abs", c_function);
 
+    //FunctionSymbol Class
+    c_function_symbol = rb_define_class_under(m_symengine, "FunctionSymbol", c_function);
+    rb_define_method(c_function_symbol, "initialize", cfunction_functionsymbol_init, -2);
+
     //TrigFunction SubClasses
     c_sin = rb_define_class_under(m_symengine, "Sin", c_trig_function);
     c_cos = rb_define_class_under(m_symengine, "Cos", c_trig_function);
diff --git a/ext/symengine/symengine.h b/ext/symengine/symengine.h
index 20dfcc0..f850d57 100644
--- a/ext/symengine/symengine.h
+++ b/ext/symengine/symengine.h
@@ -21,10 +21,12 @@ VALUE c_real_mpfr;
 VALUE c_complex_mpc;
 #endif //HAVE_SYMENGINE_MPC
 VALUE c_constant;
+VALUE c_subs;
 VALUE c_add;
 VALUE c_mul;
 VALUE c_pow;
 VALUE c_function;
+VALUE c_function_symbol;
 VALUE c_trig_function;
 VALUE c_hyperbolic_function;
 VALUE c_lambertw;
diff --git a/ext/symengine/symengine_utils.c b/ext/symengine/symengine_utils.c
index cf4f015..e4385f1 100644
--- a/ext/symengine/symengine_utils.c
+++ b/ext/symengine/symengine_utils.c
@@ -8,7 +8,6 @@ void sympify(VALUE operand2, basic_struct *cbasic_operand2) {
     VALUE a, b;
     double f;
     char *c;
-    rb_cBigDecimal = CLASS_OF(rb_eval_string("BigDecimal.new('0.0001')"));
 
     switch(TYPE(operand2)) {
         case T_FIXNUM:
@@ -63,7 +62,7 @@ void sympify(VALUE operand2, basic_struct *cbasic_operand2) {
         case T_DATA:
             c = rb_obj_classname(operand2);
             #ifdef HAVE_SYMENGINE_MPFR
-            if(CLASS_OF(operand2) == rb_cBigDecimal){
+            if (strcmp(c, "BigDecimal") == 0) {
                 c = RSTRING_PTR( rb_funcall(operand2, rb_intern("to_s"), 1, rb_str_new2("F")) );
                 real_mpfr_set_str(cbasic_operand2, c, 200);
                 break;
@@ -113,12 +112,16 @@ VALUE Klass_of_Basic(const basic_struct *basic_ptr) {
         #endif //HAVE_SYMENGINE_MPFR
         case SYMENGINE_CONSTANT:
             return c_constant;
+        case SYMENGINE_SUBS:
+            return c_subs;
         case SYMENGINE_ADD:
             return c_add;
         case SYMENGINE_MUL:
             return c_mul;
         case SYMENGINE_POW:
             return c_pow;
+        case SYMENGINE_FUNCTIONSYMBOL:
+            return c_function_symbol;
         case SYMENGINE_ABS:
             return c_abs;
         case SYMENGINE_SIN:
diff --git a/lib/symengine.rb b/lib/symengine.rb
index 21d4c7d..e4d3477 100644
--- a/lib/symengine.rb
+++ b/lib/symengine.rb
@@ -1,6 +1,5 @@
 module SymEngine
   class << self
-
     # Defines a shortcut for SymEngine::Symbol.new() allowing multiple symbols
     # to be created all at once.
     #
@@ -27,7 +26,10 @@ def symbols ary_or_string, *params
       ary_or_string.map do |symbol_or_string| 
         SymEngine::Symbol.new(symbol_or_string)
       end
-    end
+    end    
+    def Function(n)
+      return SymEngine::UndefFunction.new(n)
+    end    
   end
 end
 
@@ -37,3 +39,4 @@ def symbols ary_or_string, *params
 require 'symengine/integer'
 require 'symengine/complex'
 require 'symengine/complex_double'
+require 'symengine/undef_function'
diff --git a/lib/symengine/undef_function.rb b/lib/symengine/undef_function.rb
new file mode 100644
index 0000000..8982b8b
--- /dev/null
+++ b/lib/symengine/undef_function.rb
@@ -0,0 +1,13 @@
+module SymEngine
+  class UndefFunction
+    
+    def initialize(n)
+        @name = n
+    end
+    
+    def call(*args)
+        SymEngine::FunctionSymbol.new(@name, *args)
+    end
+    
+  end
+end
diff --git a/spec/function_symbol_spec.rb b/spec/function_symbol_spec.rb
new file mode 100644
index 0000000..f73d2eb
--- /dev/null
+++ b/spec/function_symbol_spec.rb
@@ -0,0 +1,40 @@
+describe SymEngine::FunctionSymbol do
+
+  let(:x) { sym('x') }
+  let(:y) { sym('y') }
+  let(:z) { sym('z') }
+
+  describe '.new' do
+    context 'with symbols' do
+      subject { SymEngine::FunctionSymbol.new('f', x, y, z) }
+      it { is_expected.to be_a SymEngine::FunctionSymbol }
+    end
+
+    context 'with compound arguments' do
+      subject { SymEngine::FunctionSymbol.new('f', 2*x, y, SymEngine::sin(z)) }
+      it { is_expected.to be_a SymEngine::FunctionSymbol }
+    end
+  end
+  
+  context '#diff' do
+    let(:fun) { (SymEngine::FunctionSymbol.new('f', 2*x, y, SymEngine::sin(z))) }
+    context 'by variable' do
+      subject { fun.diff(x)/2 }
+      it { is_expected.to be_a SymEngine::Subs }
+    end
+  end
+  
+  context 'Initializing with UndefFunctions' do
+    let(:fun) { SymEngine::Function('f') }
+    context 'UndefFunction' do
+      subject { fun }
+      it { is_expected.to be_a SymEngine::UndefFunction }
+    end   
+    context 'using call method for UndefFunction' do
+      subject { fun.(x, y, z) }
+      it { is_expected.to be_a SymEngine::FunctionSymbol }
+    end
+  end
+end
+
+