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 A072277 #26 Jul 06 2021 02:37:14
%S A072277 10,25,51,91,148,225,325,451,606,793,1015,1275,1576,1921,2313,2755,
%T A072277 3250,3801,4411,5083,5820,6625,7501,8451,9478,10585,11775,13051,14416,
%U A072277 15873,17425,19075,20826,22681,24643,26715,28900,31201,33621,36163
%N A072277 Smallest integer > 1 which is both n-gonal and centered n-gonal.
%C A072277 a(n) is the (n-1)-th centered n-gonal number. The n-th centered n-gonal number is A100119(n) and the (n+1)-th centered n-gonal number is A158842(n). - _Mohammed Yaseen_, Jun 06 2021
%H A072277 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,-6,4,-1).
%F A072277 a(n) = (n^3 - n^2 + 2)/2.
%F A072277 a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4), with a(3)=10, a(4)=25, a(5)=51, a(6)=91. - _Harvey P. Dale_, Aug 19 2011
%F A072277 G.f.: x^3*(-3*x^3 + 11*x^2 - 15*x + 10)/(x-1)^4. - _Harvey P. Dale_, Aug 19 2011
%e A072277 a(4) = 25 is both square and centered square.
%t A072277 LinearRecurrence[{4,-6,4,-1},{10,25,51,91},50] (* or *) Table[(n^3-n^2+ 2)/2,{n,3,50}] (* _Harvey P. Dale_, Aug 19 2011 *)
%Y A072277 Cf. A100119, A158842, A162607.
%K A072277 nonn,easy
%O A072277 3,1
%A A072277 _David W. Wilson_, Jul 09 2002