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A218826 Number of indecomposable (by concatenation) alternating n-anagrams. 3
1, 1, 4, 25, 217, 2470, 35647, 637129, 13843948, 360022957, 11054457253, 396003680518, 16377463914091, 774714094061221, 41572230979229284, 2512149910125036865, 169831839578092130017, 12769241823369505582150, 1062122471116082751430087, 97264621940872013476357969 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Alternating anagrams enumeration is related to A000366 by a(n) = A000366(n+1).

For all n, a(n) are periodically congruent to 1, 1 and 4 modulo 6.

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..200

G. Kreweras and J. Barraud, Anagrammes altern├ęs, European Journal of Combinatorics,Volume 18, Issue 8, November 1997, Pages 887-891.

G. Kreweras and D. Dumont, Sur les anagrammes altern├ęs, Discrete Mathematics, Volume 211, Issues 1-3, 28 January 2000, Pages 103-110.

FORMULA

G.f.: (1-Q(0))/x, where Q(k)= 1 - 1/2*(k+1)*(k+2)*x/(1 - 1/2*(k+1)*(k+2)*x/Q(k+1)); (continued fraction). - Sergei N. Gladkovskii, May 03 2013

G.f.: 1/Q(0), where Q(k) = 1 - (k+1)*(k+2)/2*x/(1 - (k+2)*(k+3)/2*x/Q(k+1) ; (continued fraction). - Sergei N. Gladkovskii, Nov 17 2013

a(n) = A000366(n + 1) - Sum_{k=2..n} A239894(n, k). - Andrew Howroyd, Feb 25 2020

PROG

(PARI) a000366(n)= {return((-1/2)^(n-2)*sum(k=0, n, binomial(n, k)*(1-2^(n+k+1))*bernfrac(n+k+1))); }

bi(n, k) = {if (matb[n, k] == 0, if (n==k, v=1, if (k==1, v = b(n), v = sum(i=1, n-k+1, b(i)*bi(n-i, k-1)); ); ); matb[n, k] = v; ); return (matb[n, k]); }

b(n) = {if (n==1, return(a000366(n+1)), return(a000366(n+1) - sum(i=2, n, bi(n, i)))); }

allb(m) = {matb = matrix(m, m); for (i=1, m, print1(b(i), ", "); ); }

CROSSREFS

Cf. A000366, A218827. First column of A239894.

Sequence in context: A198198 A007830 A305404 * A060911 A060912 A209319

Adjacent sequences:  A218823 A218824 A218825 * A218827 A218828 A218829

KEYWORD

nonn

AUTHOR

Michel Marcus, Nov 07 2012

STATUS

approved

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Last modified April 9 22:06 EDT 2020. Contains 333371 sequences. (Running on oeis4.)