TGammaDerivative = class sealed(TSimpleFunctionalDerivative)
protected
  function GetFunctionName(): string; override;
  function BaseDerivative(argument: TBaseExpression): TBaseExpression; override;
public
  class function IsRealized: boolean; override;
end;

function TGammaDerivative.GetFunctionName: string;
begin
  result:= 'G';
end;

class function TGammaDerivative.IsRealized: boolean;
begin
  result:= true;
end;

function TGammaDerivative.BaseDerivative(argument: TBaseExpression): TBaseExpression;
var
  ge: TBaseExpression;
  dge: TBaseExpression;
begin
  ge:= TFunctionExpression.CreateSimple('G', argument);
  dge:= TFunctionExpression.CreateSimple('Y', argument);

  result:= TProductExpression.MakeProduct(ge, dge);
end;