A220896 Number of closed lambda-terms of size n with at most 2 free de Bruijn indices.

2, 7, 41, 312, 2784, 27917, 307943, 3690055, 47635777, 658405747, 9695617821, 151488900012, 2502346785164, 43560247035581, 796828655891895, 15277217956632472, 306268455537216444, 6406216616616309707, 139535240869213561821, 3159084189417388736206

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-François Alcover, May 23 2017 *)

Crossrefs

Sequence in context: A215207 A106871 A107376 * A087804 A378325 A006677

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