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 A099947 Number of topologically connected set partitions of {1,...,n}. 37
 1, 1, 1, 1, 2, 6, 21, 85, 385, 1907, 10205, 58455, 355884, 2290536, 15518391, 110283179, 819675482, 6355429550, 51293023347, 430062712439, 3739408304962, 33665192703946, 313354708842791, 3011545611755271, 29847401178719637, 304713973031878687, 3201007359886598431 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS A set partition of {1,...,n} is topologically connected if the graph whose vertices are the blocks and whose edges are crossing pairs of blocks is connected, where two blocks cross each other if they are of the form {{...x...y...}, {...z...t...}} for some x < z < y < t or z < x < t < y. - Gus Wiseman, Feb 19 2019 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..222 Janet Simpson Beissinger, The enumeration of irreducible combinatorial objects, J. Comb. Theory, Ser. A, 38 (1985), pp. 143-169. (Example 6.2) Daniel Birmajer, Juan B. Gil, Michael D. Weiner, A family of Bell transformations, arXiv:1803.07727 [math.CO], 2018. Kenneth J. Dykema, Generating functions for purely crossing partitions, arXiv:1602.03469 [math.CO], 2016. See column NIS in Table 2 p. 8. FindStat, The number of topologically connected components of a set partition. Aoife Hennessy, A Study of Riordan Arrays with Applications to Continued Fractions, Orthogonal Polynomials and Lattice Paths, Ph. D. Thesis, Waterford Institute of Technology, Oct. 2011. M. Klazar, Bell numbers, their relatives and algebraic differential equations M. Klazar, Bell numbers, their relatives and algebraic differential equations, J. Combin. Theory, A 102 (2003), 63-87. FORMULA From Paul D. Hanna, Apr 16 2013: (Start) O.g.f. A(x) satisfies (1) A(x) = Sum_{n>=0} A000110(n)*x^n/A(x)^n, where A000110 are the Bell numbers. (2) A(x) = Sum_{n>=0} x^n / Product_{k=1..n} (A(x) - k*x). (3) A(x) = 1/(1 - x/(A(x) - 1*x/(1 - x/(A(x) - 2*x/(1 - x/(A(x) - 3*x/(1 - x/(A(x) - 4*x/(1 - x/(A(x) - ... )))))))), a continued fraction. (End) B(n) = Sum_p Product_{s in p} a(|s|) where p is a non-crossing set partition of {1,...,n} and B = A000110. In words, every set partition of {1,...,n} can be uniquely decomposed as a non-crossing set partition together with a topologically connected set partition of each block. - Gus Wiseman, Feb 19 2019 EXAMPLE O.g.f.: A(x) = 1 + x + x^2 + x^3 + 2*x^4 + 6*x^5 + 21*x^6 + 85*x^7 +... From Paul D. Hanna, Apr 16 2013]: (Start) The o.g.f. satisfies (1) A(x) = 1 + x/A(x) + 2*x^2/A(x)^2 + 5*x^3/A(x)^3 + 15*x^4/A(x)^4 + 52*x^5/A(x)^5 + 203*x^6/A(x)^6 + ... + A000110(n)*x^n/A(x)^n + ... (2) A(x) = 1 + x/(A(x)-x) + x^2/((A(x)-x)*(A(x)-2*x)) + x^3/((A(x)-x)*(A(x)-2*x)*(A(x)-3*x)) + x^4/((A(x)-x)*(A(x)-2*x)*(A(x)-3*x)*(A(x)-4*x)) + ... (End) From Gus Wiseman, Feb 19 2019: (Start) The a(1) = 1 through a(6) = 21 topologically connected set partitions: {{1}} {{12}} {{123}} {{1234}} {{12345}} {{123456}} {{13}{24}} {{124}{35}} {{1235}{46}} {{13}{245}} {{124}{356}} {{134}{25}} {{1245}{36}} {{135}{24}} {{1246}{35}} {{14}{235}} {{125}{346}} {{13}{2456}} {{134}{256}} {{1345}{26}} {{1346}{25}} {{135}{246}} {{1356}{24}} {{136}{245}} {{14}{2356}} {{145}{236}} {{146}{235}} {{15}{2346}} {{13}{25}{46}} {{14}{25}{36}} {{14}{26}{35}} {{15}{24}{36}} (End) MATHEMATICA a[0] = 1; a[n_] := Module[{A = 1 + x}, For[i = 1, i <= n, i++, A = Sum[x^m/Product[A - k*x + x*O[x]^n, {k, 1, m}], {m, 0, n}]]; Coefficient[A, x^n]]; Table[a[n], {n, 0, 24}] (* Jean-François Alcover, Sep 13 2013, after Paul D. Hanna *) nn=8; nonXQ[stn_]:=!MatchQ[stn, {___, {___, x_, ___, y_, ___}, ___, {___, z_, ___, t_, ___}, ___}/; x

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