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 A178165 Number of unordered collections of distinct nonempty subsets of an n-element set where each element appears in at most 2 subsets. 4
 1, 2, 8, 59, 652, 9736, 186478, 4421018, 126317785, 4260664251, 166884941780, 7489637988545, 380861594219460, 21739310882945458, 1381634777325000263, 97089956842985393297, 7497783115765911443879, 632884743974716421132084 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS If each element must appear in exactly 1 subset, then we get the Bell numbers A000110. If each element must appear in exactly 2 subsets, then we get A002718. LINKS Alois P. Heinz, Table of n, a(n) for n = 0..300 FORMULA Binomial transform of A094574: a(n) = Sum_{k=0..n} C(n,k)*A094574(k). MATHEMATICA terms = m = 30; a094577[n_] := Sum[Binomial[n, k]*BellB[2n-k], {k, 0, n}]; egf = Exp[(1 - Exp[x])/2]*Sum[a094577[n]*(x/2)^n/n!, {n, 0, m}] + O[x]^m; A094574 = CoefficientList[egf + O[x]^m, x]*Range[0, m-1]!; a[n_] := Sum[Binomial[n, k]*A094574[[k+1]], {k, 0, n}]; Table[a[n], {n, 0, m-1}] (* Jean-François Alcover, May 24 2019 *) PROG (Python) def powerSet(k): return [toBinary(n, k) for n in range(1, 2**k)] def courcelle(maxUses, remainingSets, exact=False):     if exact and not all(maxUses<=sum(remainingSets)): ans=0     elif len(remainingSets)==0: ans=1     else:         set0=remainingSets[0]         if all(set0<=maxUses): ans=courcelle(maxUses-set0, remainingSets[1:], exact=exact)         else: ans=0         ans+=courcelle(maxUses, remainingSets[1:], exact=exact)     return ans for i in range(10):     print(i, courcelle(array([2]*i), powerSet(i), exact=False)) CROSSREFS Row n=2 of A330964. Cf. A094574, A000110, A002718, A178171, A178173. Sequence in context: A162065 A241329 A346065 * A214872 A197937 A205076 Adjacent sequences:  A178162 A178163 A178164 * A178166 A178167 A178168 KEYWORD nonn AUTHOR Daniel E. Loeb, Dec 16 2010 EXTENSIONS Edited and corrected by Max Alekseyev, Dec 19 2010 STATUS approved

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Last modified August 4 11:01 EDT 2021. Contains 346447 sequences. (Running on oeis4.)