A276681 Number of divisors of the n-th octagonal number.

1, 4, 4, 8, 4, 12, 4, 10, 9, 16, 4, 16, 4, 20, 8, 12, 6, 24, 8, 16, 8, 16, 4, 40, 6, 16, 8, 16, 8, 40, 8, 14, 8, 24, 8, 24, 4, 24, 16, 20, 6, 32, 4, 32, 24, 20, 4, 24, 12, 24, 8, 32, 4, 56, 8, 20, 12, 16, 12, 32, 4, 20, 24, 32, 8, 48, 4, 16, 16, 48, 4, 30, 8

Offset: 1

Author

Colin Barker, Sep 13 2016

Keywords

• nonn

Examples

			a(4) = 8 because the 4th octagonal number is 40, which has 8 divisors: 1,2,4,5,8,10,20,40.
		

Links

• Colin Barker, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000005, A000567.

Cf. A063440 (m=3), A048691 (m=4), A276678 (m=5), A276679 (m=6), A276680 (m=7), A276682 (m=9), A276683 (m=10).

Programs

PARI

pg(m, n) = (n^2*(m-2)-n*(m-4))/2 \\ n-th m-gonal number
vector(150, n, numdiv(pg(8,n)))

Formula

a(n) = A000005(A000567(n)).