351142domain: NAppears in sequencesPentagonal numbers (A000326) that are the product of 2 palindromes greater than 1.at n=32A115745a(n) = n^2*(3*n^2 - 1)/2.at n=22A260810a(n) = Sum_{1 <= x_1, x_2, x_3 <= n} sigma( n/gcd(x_1, x_2, x_3, n) ).at n=21A373130a(n) = Sum_{1 <= x_1, x_2 <= n} sigma( ( n/gcd(x_1, x_2, n) )^2 ).at n=21A373132