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 A273526 Number of 123-avoiding indecomposable permutations. 8
 1, 1, 3, 11, 38, 127, 423, 1423, 4854, 16787, 58776, 208001, 742888, 2674427, 9694831, 35357655, 129644774, 477638683, 1767263172, 6564120401, 24466267000, 91482563619, 343059613628, 1289904147301, 4861946401428, 18367353072127, 69533550915978, 263747951750333, 1002242216651340, 3814986502092275 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Indranil Ghosh, Table of n, a(n) for n = 1..1000 A. L. L. Gao, S. Kitaev, P. B. Zhang. On pattern avoiding indecomposable permutations, arXiv:1605.05490 [math.CO], 2016. Murray Tannock, Equivalence classes of mesh patterns with a dominating pattern, MSc Thesis, Reykjavik Univ., May 2016. FORMULA a(n) = C(n) - n + 1, where C(n) is the Catalan number. MATHEMATICA a[n_] := CatalanNumber[n] - (n - 1) Table[a[n], {n, 1, 30}] (* Alice L.L. Gao, May 24 2016 *) PROG (PARI) a(n) = binomial(2*n, n)/(n+1) - n + 1; \\ Indranil Ghosh, Mar 04 2017 (Python) import math f=math.factorial def C(n, r): return f(n)/f(r)/f(n-r) def Catalan(n): return C(2*n, n)/(n+1) def A273526(n): return Catalan(n)-n+1 # Indranil Ghosh, Mar 04 2017 CROSSREFS Cf. A000108, A003319. Sequence in context: A026361 A006189 A092201 * A026943 A307571 A047096 Adjacent sequences:  A273523 A273524 A273525 * A273527 A273528 A273529 KEYWORD nonn AUTHOR Alice L.L. Gao and Philip B. Zhang, May 24 2016 STATUS approved

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Last modified October 17 15:01 EDT 2019. Contains 328116 sequences. (Running on oeis4.)