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 A273220 a(n) = 8n^2 - 12n + 1. 1
 9, 37, 81, 141, 217, 309, 417, 541, 681, 837, 1009, 1197, 1401, 1621, 1857, 2109, 2377, 2661, 2961, 3277, 3609, 3957, 4321, 4701, 5097, 5509, 5937, 6381, 6841, 7317, 7809, 8317, 8841, 9381, 9937, 10509, 11097, 11701, 12321, 12957, 13609, 14277, 14961, 15661 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS Sequence may be obtained by starting with the segment (9, 37) followed by the line from 37 in the direction 37, 81,... in the square spiral whose vertices are the generalized hexagonal numbers (A000217). - Omar E. Pol, Jun 26 2016 LINKS Colin Barker, Table of n, a(n) for n = 2..1000 Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA From Colin Barker, May 18 2016: (Start) a(n) = 3*a(n-1)-3*a(n-2)+a(n-3) for n>4. G.f.: x^2*(9+10*x-3*x^2) / (1-x)^3. (End) MATHEMATICA Table[8 n^2 - 12 n + 1, {n, 2, 45}] (* or *) Drop[#, 2] &@ CoefficientList[Series[x^2 (9 + 10 x - 3 x^2)/(1 - x)^3, {x, 0, 45}], x] (* Michael De Vlieger, Jun 26 2016 *) PROG (PARI) Vec(x^2*(9+10*x-3*x^2)/(1-x)^3 + O(x^50)) \\ Colin Barker, May 18 2016 CROSSREFS Sequence in context: A137184 A153244 A200774 * A022276 A171443 A341403 Adjacent sequences:  A273217 A273218 A273219 * A273221 A273222 A273223 KEYWORD nonn,easy AUTHOR Dimitri Boscainos, May 18 2016 STATUS approved

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Last modified July 25 23:39 EDT 2021. Contains 346294 sequences. (Running on oeis4.)