This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A381109 #44 Mar 07 2025 21:30:08
%S A381109 1,16,52,109,187,286,406,547,709,892,1096,1321,1567,1834,2122,2431,
%T A381109 2761,3112,3484,3877,4291,4726,5182,5659,6157,6676,7216,7777,8359,
%U A381109 8962,9586,10231,10897,11584,12292,13021,13771,14542,15334,16147,16981,17836,18712
%N A381109 a(n) = (21*n^2 + 9*n + 2)/2.
%C A381109 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.
%H A381109 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%e A381109 Illustration of initial terms:
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%e A381109 1 16 52 109
%t A381109 LinearRecurrence[{3,-3,1},{1,16,52},43] (* _James C. McMahon_, Mar 07 2025 *)
%Y A381109 Cf. A003215, A005892, A381424.
%K A381109 nonn,easy
%O A381109 0,2
%A A381109 _Aaron David Fairbanks_, Mar 06 2025