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A273526
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Number of 123-avoiding indecomposable permutations.
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8
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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
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OFFSET
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1,3
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LINKS
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FORMULA
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a(n) = C(n) - n + 1, where C(n) is the Catalan number.
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MATHEMATICA
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a[n_] := CatalanNumber[n] - (n - 1)
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PROG
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(PARI)
(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)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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