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 A025551 a(n) = 3^n*(3^n + 1)/2. 3
 1, 6, 45, 378, 3321, 29646, 266085, 2392578, 21526641, 193720086, 1743421725, 15690618378, 141215033961, 1270933711326, 11438398618965, 102945573221778, 926510115949281, 8338590914403366, 75047317842209805, 675425859417626778 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS G. C. Greubel, Table of n, a(n) for n = 0..750 Index entries for linear recurrences with constant coefficients, signature (12, -27). FORMULA From Philippe Deléham, Jul 11 2005: (Start) Binomial transform of A081342. 6th binomial transform of (1, 0, 9, 0, 81, 0, 729, 0, . . ). Inverse binomial transform of A081343. a(n) = 12*a(n-1) - 27*a(n-2), a(0) = 1, a(1) = 6. G.f.: (1-6*x)/((1-3*x)*(1-9*x)). E.g.f.: exp(7*x)*cosh(3*x). (End) a(n) = ((6+sqrt(9))^n + (6-sqrt(9))^n)/2. - Al Hakanson (hawkuu(AT)gmail.com), Dec 08 2008 a(n) = Sum_{k=1..3^n} k. - Joerg Arndt, Sep 01 2013 MAPLE seq( binomial(3^n +1, 2), n=0..20); # G. C. Greubel, Jan 08 2020 MATHEMATICA LinearRecurrence[{12, -27}, {1, 6}, 20] (* G. C. Greubel, Jan 08 2020 *) PROG (PARI) Vec( (1-6*x)/((1-3*x)*(1-9*x)) + O(x^66) ) \\ Joerg Arndt, Sep 01 2013 (MAGMA) [Binomial(3^n+1, 2): n in [0..20]]; // G. C. Greubel, Jan 08 2020 (Sage) [binomial(3^n+1, 2) for n in (0..20)] # G. C. Greubel, Jan 08 2020 (GAP) List([0..20], n-> Binomial(3^n+1, 2) ); # G. C. Greubel, Jan 08 2020 CROSSREFS Sequence in context: A007193 A153399 A007194 * A101600 A233668 A243694 Adjacent sequences:  A025548 A025549 A025550 * A025552 A025553 A025554 KEYWORD nonn AUTHOR STATUS approved

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Last modified July 25 19:05 EDT 2021. Contains 346291 sequences. (Running on oeis4.)