def _add(self, other):
        if isinstance(other, SeqPer):
            per1, lper1 = self.periodical, self.period
            per2, lper2 = other.periodical, other.period

            per_length = lcm(lper1, lper2)

            new_per = []
            for x in range(per_length):
                ele1 = per1[x % lper1]
                ele2 = per2[x % lper2]
                new_per.append(ele1 + ele2)

            start, stop = self._intersect_interval(other)
            return SeqPer(new_per, (self.variables[0], start, stop))
See docstring of SeqBase._add
dataset/ETHPy150Open sympy/sympy/sympy/series/sequences.py/SeqPer._add