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 A253880 Triangular numbers (A000217) that are also centered heptagonal numbers (A069099). 6
 1, 253, 64261, 16322041, 4145734153, 1053000152821, 267457893082381, 67933251842771953, 17254778510170993681, 4382645808331589623021, 1113174780537713593253653, 282742011610770921096804841, 71815357774355276244995175961, 18240818132674629395307677889253 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Colin Barker, Table of n, a(n) for n = 1..416 Index entries for linear recurrences with constant coefficients, signature (254,-1). FORMULA a(n) = 254*a(n-1) - a(n-2). G.f.: -x*(x-1) / (x^2 - 254*x + 1). EXAMPLE 253 is in the sequence because it is the 22nd triangular number and the 9th centered heptagonal number. MATHEMATICA LinearRecurrence[{254, -1}, {1, 253}, 20] (* Harvey P. Dale, May 17 2017 *) PROG (PARI) Vec(-x*(x-1)/(x^2-254*x+1) + O(x^100)) CROSSREFS Cf. A000217, A069099, A253878, A253879. Similar sequences of the type cosh((2*m+1)*arccosh(k))/k are listed in A302329. This is the case k=8. Sequence in context: A176087 A006060 A077695 * A145628 A237419 A243666 Adjacent sequences:  A253877 A253878 A253879 * A253881 A253882 A253883 KEYWORD nonn,easy AUTHOR Colin Barker, Jan 17 2015 STATUS approved

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Last modified November 25 11:18 EST 2020. Contains 338623 sequences. (Running on oeis4.)