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 A027637 a(n) = Product_{i=1..n} (4^i - 1). 19
 1, 3, 45, 2835, 722925, 739552275, 3028466566125, 49615367752825875, 3251543125681443718125, 852369269595510700600441875, 893773106866112632882108339078125, 3748755223447856814435325652920396921875 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The q-analog of double factorials (A000165) evaluated at q=2. - Michael Somos, Sep 12 2014 3^n*5^(floor(n/2))|a(n) for n>=1. - G. C. Greubel, Nov 21 2015 Given probability p = 1/4^n that an outcome will occur at the n-th stage of an infinite process, then starting at n=1, 1-a(n)/A053763(n+1) is the probability that the outcome has occurred up to and including the n-th iteration. The limiting ratio is 1-A100221 ~ 0.3114625. - Bob Selcoe, Mar 01 2016 LINKS G. C. Greubel, Table of n, a(n) for n = 0..50 FORMULA a(n) ~ c * 2^(n*(n+1)), where c = Product_{k>=1} (1-1/4^k) = A100221 = 0.688537537120339715456514357293508184675549819378... . - Vaclav Kotesovec, Nov 21 2015 Equals 4^(binomial(n+1,2))*(1/4;1/4)_{n}, where (a;q)_{n} is the q-Pochhammer symbol. - G. C. Greubel, Dec 24 2015 G.f.: Sum_{n>=0} 4^(n*(n+1)/2)*x^n / Product_{k=0..n} (1 + 4^k*x). - Ilya Gutkovskiy, May 22 2017 MAPLE A027637 := proc(n)     mul( 4^i-1, i=1..n) ; end proc: seq(A027637(n), n=0..8) ; MATHEMATICA A027637 = Table[Product[4^i - 1, {i, n}], {n, 0, 9}] (* Alonso del Arte, Nov 14 2011 *) a[ n_] := If[ n < 0, 0, Product[ (q^(2 k) - 1) / (q - 1), {k, n}] /. q -> 2]; (* Michael Somos, Sep 12 2014 *) Abs@QPochhammer[4, 4, Range[0, 10]] (* Vladimir Reshetnikov, Nov 20 2015 *) PROG (PARI) a(n) = prod(i=1, n, 4^i-1); \\ Michel Marcus, Nov 21 2015 (MAGMA) [1] cat [&*[4^k-1: k in [1..n]]: n in [1..11]]; // Vincenzo Librandi, Dec 24 2015 CROSSREFS Cf. A000165. Cf. A005329 (q=2), A027871 (q=3), A027872 (q=5), A027873 (q=6), A027875 (q=7), A027876 (q=8), A027877 (q=9), A027878 (q=10), A027879 (q=11), A027880 (q=12). Cf. A053763, A100221. Sequence in context: A012833 A012747 A210933 * A228903 A198952 A099168 Adjacent sequences:  A027634 A027635 A027636 * A027638 A027639 A027640 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified May 21 05:37 EDT 2022. Contains 353889 sequences. (Running on oeis4.)