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A220896 Number of closed lambda-terms of size n with at most 2 free de Bruijn indices. 2
2, 7, 41, 312, 2784, 27917, 307943, 3690055, 47635777, 658405747, 9695617821, 151488900012, 2502346785164, 43560247035581, 796828655891895, 15277217956632472, 306268455537216444, 6406216616616309707, 139535240869213561821, 3159084189417388736206 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..19.

Katarzyna Grygiel and Pierre Lescanne, Counting and generating lambda-terms, arXiv preprint arXiv:1210.2610, 2012

FORMULA

Grygiel and Lescanne give a recurrence.

MATHEMATICA

T[0, m_] := m; T[n_, m_] := T[n, m] = T[n-1, m+1] + Sum[T[i, m] T[n-i-1, m], {i, 0, n-1}];

a[n_] := T[n, 2];

Table[a[n], {n, 0, 14}] (* Jean-Fran├žois Alcover, May 23 2017 *)

CROSSREFS

Sequence in context: A215207 A106871 A107376 * A087804 A006677 A346271

Adjacent sequences:  A220893 A220894 A220895 * A220897 A220898 A220899

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Dec 31 2012

EXTENSIONS

More terms from Alois P. Heinz, May 23 2017

STATUS

approved

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Last modified September 23 20:42 EDT 2021. Contains 347617 sequences. (Running on oeis4.)