155156domain: NAppears in sequencesNumber of 3 X 3 integer matrices with elements in the range [ -n,n ] which represent a three-fold rotation.at n=6A053172a(1) = 2; a(n) = a(n-1)-th even nontotient number.at n=7A072415Indices of 7-gonal numbers which are also centered 7-gonal numbers.at n=5A133271