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A220896
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Number of closed lambda-terms of size n with at most 2 free de Bruijn indices.
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2
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2, 7, 41, 312, 2784, 27917, 307943, 3690055, 47635777, 658405747, 9695617821, 151488900012, 2502346785164, 43560247035581, 796828655891895, 15277217956632472, 306268455537216444, 6406216616616309707, 139535240869213561821, 3159084189417388736206
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OFFSET
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0,1
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LINKS
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FORMULA
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Grygiel and Lescanne give a recurrence.
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MATHEMATICA
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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];
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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