The On-Line Encyclopedia of Integer Sequences (OEIS)

A381109 revision #41

A381109

a(n) = (21*n^2 + 9*n + 2)/2.

1, 16, 52, 109, 187, 286, 406, 547, 709, 892, 1096, 1321, 1567, 1834, 2122, 2431, 2761, 3112, 3484, 3877, 4291, 4726, 5182, 5659, 6157, 6676, 7216, 7777, 8359, 8962, 9586, 10231, 10897, 11584, 12292, 13021, 13771, 14542, 15334, 16147, 16981, 17836, 18712

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OFFSET

0,2

COMMENTS

A 9-sided figure is obtained by truncating every other corner of a centered hexagonal figure as in A003215; a(n) is the number of dots in such a 9-sided figure with n+1 dots on each side.

LINKS

Table of n, a(n) for n=0..42.

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

EXAMPLE

Illustration of initial terms:

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