def __mul__(self,other):
        """multiplicerer 2 matricer når antallet af søjler i den første er lig antallet af rækker i den næste."""
        if self.columns == other.rows :
            colself = []
            rowother = other.matrix
            a= self.columns
            b= other.rows
            for x in xrange(self.columns):
                colself.append([])
                for y in xrange(self.rows):
                    colself[x].append(self.matrix[y][x])

            mulmatrix  = SimpleMatrix(a,b)

            for i in xrange(mulmatrix.rows):
                for j in xrange(mulmatrix.columns):
                    counter =0
                    for k in xrange(self.rows):
                        counter += colself[j][k]*other.matrix[i][k]
                    mulmatrix.matrix[i][j] = counter
            return mulmatrix