(1-aq)⁢(1-bq)⁢∑k=0n(a⁢pk,b⁢p-k;q)n-1⁢(1-(a⁢p2⁢k/b))(p;p)n⁢(p;p)n-k⁢(a⁢pk/b;q)n+1⁢(-1)k⁢p(k2)=δn,0.