230686720domain: NAppears in sequencesNumber of subsets of {1,.., n} containing exactly two primes.at n=32A089822a(n) = n*4^(n/2 - 1)*(9 + (-1)^n).at n=22A187274a(n) = coefficient of x^n in A(x) such that C(x)^2 + S(x)^2 = 1 where: C(x) + i*S(x) = Sum_{n=-oo..+oo} i^n * (2*x)^(n^2) * A(x)^n.at n=23A357787