A220895 Number of closed lambda-terms of size n with at most 1 free de Bruijn index.

1, 3, 13, 76, 542, 4493, 42131, 439031, 5020105, 62382279, 835980065, 12004984120, 183754242626, 2984264710781, 51220227153987, 925859588215096, 17571969962025822, 349210371834021295, 7249185099929421649, 156847124089924073770, 3530125495029323284404

Offset

0,2

Links

Vincenzo Librandi, Table of n, a(n) for n = 0..200

Katarzyna Grygiel and Pierre Lescanne, Counting and generating lambda-terms, arXiv preprint arXiv:1210.2610 [cs.LO], 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, 1];
Table[a[n], {n, 0, 14}] (* Jean-François Alcover, May 23 2017 *)

Crossrefs

Sequence in context: A047730 A162435 A059040 * A352308 A336421 A189239

Adjacent sequences: A220892 A220893 A220894 * A220896 A220897 A220898

Keyword

nonn

Author

N. J. A. Sloane, Dec 31 2012

Extensions

More terms from Alois P. Heinz, May 23 2017

Status

approved