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A026549
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Ratios of successive terms are 2,3,2,3,2,3,2,3...
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5
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1, 2, 6, 12, 36, 72, 216, 432, 1296, 2592, 7776, 15552, 46656, 93312, 279936, 559872, 1679616, 3359232, 10077696, 20155392, 60466176, 120932352, 362797056, 725594112, 2176782336, 4353564672, 13060694016, 26121388032
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| Appears to be the number of permutations p of {1,2,...,n} such that p(i)+p(i+1)>=n for every i=1,2,...,n-1 (if offset is 1). - Vladeta Jovovic (vladeta(AT)eunet.rs), Dec 15 2003
Contribution from Gary W. Adamson (qntmpkt(AT)yahoo.com), Aug 02 2010: (Start)
Equals eigensequence of a triangle with 1's in even columns and (1,3,3,3,...)
in odd columns. a(5) = 72 = (1, 3, 1, 3, 1, 1) dot (1, 1, 2, 6, 12, 36)
= (1 + 3 + 2 + 18 + 12 + 36), where (1, 3, 1, 3, 1, 1) = row 5 of the
generating triangle. (End)
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LINKS
| Vincenzo Librandi, Table of n, a(n) for n = 0..700
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FORMULA
| Equals T(n, 0) + T(n, 1) + ... + T(n, 2n), T given by A026536.
G.f.: (1+2x)/(1-6x^2) - Paul Barry (pbarry(AT)wit.ie), Aug 25 2003
a(n+3) = a(n+2)*a(n+1)/a(n). [Reinhard Zumkeller, Mar 04 2011]
a(n) = ((1/2)*(3-(-1)^n)*6^floor(n/2)), or a(n) = 6*a(n-2). [From Vincenzo Librandi, Jun 08 2011]
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PROG
| (MAGMA) [((1/2)*(3-(-1)^n)*6^Floor(n/2)) : n in [0..30]]; // Vincenzo Librandi, Jun 08 2011
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CROSSREFS
| For n>0, a(n) = 2*A026532(n). Cf. A026551, A026567.
Sequence in context: A076278 A204512 A099576 * A120766 A121404 A202337
Adjacent sequences: A026546 A026547 A026548 * A026550 A026551 A026552
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KEYWORD
| nonn
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AUTHOR
| Clark Kimberling (ck6(AT)evansville.edu)
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EXTENSIONS
| New definition from Ralf Stephan, Dec 01, 2004
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