∑ℓ=-∞∞(h~n⁡(c⁢qℓ;q)⁢h~m⁡(c⁢qℓ;q)+h~n⁡(-c⁢qℓ;q)⁢h~m⁡(-c⁢qℓ;q))⁢qℓ(-c2⁢q2⁢ℓ;q2)∞=2⁢(q2,-c2⁢q,-c-2⁢q;q2)∞(q,-c2,-c-2⁢q2;q2)∞⁢(q;q)nqn2⁢δn,m,