A065221 Fill a triangular array by rows by writing numbers 1 up to b(0), 1 up to b(1), etc., where b(n) are the pentagonal numbers. The final elements of the rows form a(n).

1, 2, 5, 4, 9, 3, 10, 18, 5, 15, 26, 3, 16, 30, 45, 10, 27, 45, 64, 14, 35, 57, 80, 12, 37, 63, 90, 1, 30, 60, 91, 123, 11, 45, 80, 116, 153, 15, 54, 94, 135, 177, 10, 54, 99, 145, 192, 240, 42, 92, 143, 195, 248, 15, 70, 126, 183, 241, 300, 30, 91, 153, 216, 280, 345, 35