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A247415 Number of friezes of type D_n. 1
1, 4, 14, 51, 187, 695, 2606, 9842, 37386, 142693, 546790, 2102312, 8106308, 31335060, 121390028, 471159761, 1831860961, 7133082300, 27813493104, 108585087657, 424396534100, 1660418620528, 6502345229958, 25485677806201, 99969379431223, 392424954930562, 1541494622610616, 6059022365002926, 23829761312067896 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..29.

B. Fontaine and P.-G. Plamondon, Counting friezes in type D_n, arXiv:1409.3698 [math.CO], 2014.

FORMULA

a(n) = sum_{m=1..n} A000005(m)*binomial(2n-m-1,n-m).

MAPLE

a:= n -> add(numtheory:-tau(m)*binomial(2*n-m-1, n-m), m=1..n):

seq(a(n), n=1..100); # Robert Israel, Sep 17 2014

MATHEMATICA

a[n_] := Sum[DivisorSigma[0, m] Binomial[2n-m-1, n m], {m, 1, n}]

Array[a, 29] (* Jean-Fran├žois Alcover, Sep 18 2018 *)

PROG

(PARI) a(n) = sum(m=1, n, numdiv(m)*binomial(2*n-m-1, n-m) ); \\ Joerg Arndt, Sep 16 2014

CROSSREFS

Cf. A000108, A247416 and A000984, the number of friezes of type A_n, B_n and C_n.

Sequence in context: A096241 A283108 A211303 * A292463 A149488 A058692

Adjacent sequences:  A247412 A247413 A247414 * A247416 A247417 A247418

KEYWORD

nonn

AUTHOR

Bruce Fontaine, Sep 16 2014

STATUS

approved

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Last modified October 23 14:11 EDT 2019. Contains 328345 sequences. (Running on oeis4.)