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 A342939 #21 Jan 14 2023 12:41:30
%S A342939 1,2,5,7,11,16,22,29,37,46,56,67,79,92,106,121,137,154,172,191,211,
%T A342939 232,254,277,301,326,352,379,407,436,466,497,529,562,596,631,667,704,
%U A342939 742,781,821,862,904,947,991,1036,1082,1129,1177,1226,1276,1327,1379,1432,1486
%N A342939 a(n) is the Skolem number of the triangular grid graph T_n.
%C A342939 For the meaning of Skolem number of a graph, see Definitions 1.4 and 1.5 in Carrigan and Green.
%H A342939 Braxton Carrigan and Garrett Green, <a href="https://research.library.kutztown.edu/contact/vol2/iss1/2/">Skolem Number of Subgraphs on the Triangular Lattice</a>, Communications on Number Theory and Combinatorial Theory 2 (2021), Article 2.
%H A342939 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A342939 O.g.f.: x*(1 - x + 2*x^2 - 3*x^3 + 3*x^4 - x^5)/(1 - x)^3.
%F A342939 E.g.f.: exp(x)*(2 + x^2)/2 - 1 + x^3/6.
%F A342939 a(n) = 3*a(n-1) - 3*a(n-2) - a(n-3) for n > 6.
%F A342939 Except for a(3) = 5:
%F A342939 a(n) = 1 + n*(n - 1)/2 (see Theorem 2.5 in Carrigan and Green).
%F A342939 a(n) = 1 + A161680(n).
%F A342939 a(n) = A152947(n-1).
%t A342939 LinearRecurrence[{3,-3,1},{1,2,5,7,11,16},55]
%Y A342939 Cf. A152947, A161680, A247476, A342938, A342940.
%Y A342939 For n > 1, 3*A002061(n) gives the Skolem number of the hexagonal grid graph H_n.
%K A342939 nonn,easy
%O A342939 1,2
%A A342939 _Stefano Spezia_, Mar 30 2021