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 A084215 G.f.: (1+x^2)/(1-2x). 9
 1, 2, 5, 10, 20, 40, 80, 160, 320, 640, 1280, 2560, 5120, 10240, 20480, 40960, 81920, 163840, 327680, 655360, 1310720, 2621440, 5242880, 10485760, 20971520, 41943040, 83886080, 167772160, 335544320, 671088640, 1342177280, 2684354560, 5368709120, 10737418240 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Associated with a math magic problem. Elements are the sums of consecutive pairs of elements of A084214. LINKS Index entries for linear recurrences with constant coefficients, signature (2). FORMULA a(n) = sum(k=0..n, 2^(n-k)*binomial(1, k/2)*(1+(-1)^k)/2 ). - Paul Barry, Oct 15 2004 a(n) = A020714(n-2), n>1. - R. J. Mathar, Dec 19 2008 a(n) is the sum of top row terms of M^n, M = an infinite square production matrix as follows: 1, 1, 0, 0, 0, 0,... 1, 1, 1, 0, 0, 0,... 0, 0, 0, 0, 0, 0,... 0, 0, 0, 0, 0, 0,... ... - Gary W. Adamson, Aug 26 2011 a(n) = floor(2^(n-2)*5). (* From Taher Jamshidi, Sep 15 2012 *) a(n) = 2*a(n-1) for n>=3, a(0) = 1, a(1) = 2, a(2) = 5. - Philippe Deléham, Mar 13 2013 EXAMPLE a(4) = 20 = (8 + 8 + 4) since the top row of M^4 = (8, 8, 4, 0, 0, 0,...). MATHEMATICA Join[{1, 2, a = 5}, Table[a = 2*a, {n, 0, 40}]] (* Vladimir Joseph Stephan Orlovsky, Jun 09 2011 *) Table[Int[2^(n-2)*5], {n, 0, 40}] (* From Taher Jamshidi, Sep 15 2012 *) CROSSREFS Cf. A060816. Sequence in context: A222082 A293324 A284904 * A024810 A049938 A002460 Adjacent sequences:  A084212 A084213 A084214 * A084216 A084217 A084218 KEYWORD easy,nonn AUTHOR Paul Barry, May 19 2003 STATUS approved

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