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 A139607 #31 Apr 16 2024 07:08:28
%S A139607 7,28,49,70,91,112,133,154,175,196,217,238,259,280,301,322,343,364,
%T A139607 385,406,427,448,469,490,511,532,553,574,595,616,637,658,679,700,721,
%U A139607 742,763,784,805,826,847,868,889,910,931,952,973,994
%N A139607 a(n) = 21*n + 7.
%C A139607 Numbers of the 7th column of positive numbers in the square array of nonnegative and polygonal numbers A139600.
%C A139607 7th transversal numbers (or 7-transversal numbers): (A000217(7)-7)*n + 7.
%D A139607 A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 189. - From _N. J. A. Sloane_, Dec 01 2012
%H A139607 Vincenzo Librandi, <a href="/A139607/b139607.txt">Table of n, a(n) for n = 0..5000</a>
%H A139607 <a href="/index/Pol#polygonal_numbers">Index to sequences related to polygonal numbers</a>
%H A139607 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A139607 a(n) = A057145(n+2,7).
%F A139607 G.f.: 7*(1+2*x)/(x-1)^2. - _R. J. Mathar_, Jul 28 2016
%F A139607 From _Elmo R. Oliveira_, Apr 12 2024: (Start)
%F A139607 E.g.f.: 7*exp(x)*(1 + 3*x).
%F A139607 a(n) = 7*A016777(n) = A008603(n) + 7 = A152744(n+1) - A152744(n).
%F A139607 a(n) = 2*a(n-1) - a(n-2) for n >= 2. (End)
%t A139607 Range[7, 1500, 21] (* _Vladimir Joseph Stephan Orlovsky_, Jun 01 2011 *)
%t A139607 21*Range[0,50]+7 (* or *) LinearRecurrence[{2,-1},{7,28},50] (* _Harvey P. Dale_, Feb 23 2020 *)
%o A139607 (Magma) [21*n+7: n in [0..60]]; // _Vincenzo Librandi_, Jul 23 2011
%o A139607 (PARI) a(n)=21*n+7 \\ _Charles R Greathouse IV_, Oct 05 2011
%Y A139607 Cf. A000217, A008603, A016777, A057145, A139600, A152744.
%K A139607 easy,nonn
%O A139607 0,1
%A A139607 _Omar E. Pol_, Apr 27 2008