16761424636domain: NAppears in sequencesIndices of pentagonal numbers which are also triangular.at n=10A046174a(-1) = a(0) = 0, a(1) = 1; a(n) = 5a(n-1) - 5a(n-2) + a(n-3) + 2*(-1)^(n+1), alternatively a(n) = 3a(n-1) + 3a(n-2) - a(n-3).at n=19A109437