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 A277996 Number of free pure symmetric multifunctions (with empty expressions allowed) with one atom and n positions. 33
 1, 1, 2, 5, 13, 36, 102, 299, 892, 2713, 8364, 26108, 82310, 261804, 838961, 2706336, 8780725, 28636157, 93818641, 308641277, 1019140129, 3376604826, 11221805968, 37399728251, 124967677989, 418564867751, 1405030366113, 4726036692421, 15927027834163, 53770343259613 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also the number of distinct orderless Mathematica expressions with one atom and n positions. LINKS Andrew Howroyd, Table of n, a(n) for n = 1..200 Mathematica Reference, Orderless. FORMULA From Ilya Gutkovskiy, Apr 30 2019: (Start) G.f. A(x) satisfies: A(x) = x * (1 + A(x) * exp(Sum_{k>=1} A(x^k)/k)). G.f.: A(x) = Sum_{n>=1} a(n)*x^n = x * (1 + (Sum_{n>=1} a(n)*x^n) * Product_{n>=1} 1/(1 - x^n)^a(n)). (End) EXAMPLE The a(5)=13 Mathematica expressions are: x[x,x,x] x[x,x][] x[x][x] x[][x,x] x[x,x[]] x[x[x]] x[x][][] x[][x][] x[][][x] x[x[]][] x[][x[]] x[x[][]] x[][][][] MATHEMATICA multing[t_, n_]:=Array[(t+#-1)/#&, n, 1, Times]; a[n_]:=a[n]=If[n===1, 1, Sum[a[k]*Sum[Product[multing[a[First[s]], Length[s]], {s, Split[p]}], {p, IntegerPartitions[n-k-1]}], {k, 1, n-1}]]; Array[a, 30] PROG (PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, 1/n))))-1, -#v)} seq(n)={my(v=[1]); for(n=2, n, my(t=EulerT(v)); v=concat(v, v[n-1] + sum(k=1, n-2, v[k]*t[n-k-1]))); v} \\ Andrew Howroyd, Aug 19 2018 CROSSREFS Cf. A000108, A001003, A005043, A052893, A279944, A280000, A317658. Sequence in context: A279196 A002844 A223096 * A370168 A099164 A358460 Adjacent sequences: A277993 A277994 A277995 * A277997 A277998 A277999 KEYWORD nonn AUTHOR Gus Wiseman, Dec 24 2016 STATUS approved

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Last modified July 24 03:05 EDT 2024. Contains 374575 sequences. (Running on oeis4.)