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 A108281 Numbers that are both triangular and pentagonal of the second kind. 3
 0, 15, 2926, 567645, 110120220, 21362755051, 4144264359690, 803965923024825, 155965244802456376, 30256453525753512135, 5869596018751378897830, 1138671371184241752666901, 220896376413724148638480980 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES L. E. Dickson, History of the Theory of Numbers. Carnegie Institute Public. 256, Washington, DC, Vol. 1, 1919; Vol. 2, 1920; Vol. 3, 1923, see vol. 2, p. 22. LINKS FORMULA a(n) = 194 * a(n-1) - a(n-2) + 16. G.f.: x^2 *(15 + x) / ((1 - x) * (1 - 194*x + x^2)). a(n) = A076139(2*n - 2) = A014979(2 - n). EXAMPLE 15*x^2 + 2926*x^3 + 567645*x^4 + 110120220*x^5 + 21362755051*x^6 + ... a(4) = 567645 which is 1065*(1065-1)/2 = 615*(3*615+1)/2. PROG (PARI) {a(n) = polchebyshev( 2*n - 2, 2, 7) / 14 + polchebyshev( 2*n - 2, 1, 7) / 84 - 1 / 12} /* Michael Somos, Jun 16 2011 */ CROSSREFS Cf. A076139, A014979. Sequence in context: A208404 A208411 A249965 * A232455 A208579 A080691 Adjacent sequences:  A108278 A108279 A108280 * A108282 A108283 A108284 KEYWORD nonn AUTHOR Michael Somos, May 30 2005 STATUS approved

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Last modified February 23 02:29 EST 2019. Contains 320411 sequences. (Running on oeis4.)