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 A022811 Number of terms in n-th derivative of a function composed with itself 3 times. 17
 1, 1, 3, 6, 13, 23, 44, 74, 129, 210, 345, 542, 858, 1310, 2004, 2996, 4467, 6540, 9552, 13744, 19711, 27943, 39452, 55172, 76865, 106200, 146173, 199806, 272075, 368247, 496642, 666201, 890602, 1184957, 1571417, 2075058, 2731677, 3582119, 4683595, 6102256 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS This also counts a restricted set of plane partitions of n. Each element of the set which contains the A000041(n) partitions of n can be converted into plane partitions by insertion of line feeds at some or all places of the "pluses." Since the length of rows in plane partitions must be nonincreasing, there are only A000041(L(P)) ways to comply with this rule, where L(P) is the number of terms in that particular partition. Example for n=4: consider all five partitions 4 = 3+1 = 2+2 = 2+1+1 = 1+1+1+1 of four. The associated a(4)=13 plane partitions are 4, 31, 3|1, 22, 2|2, 211, 21|1, 2|1|1, 1111, 111|1, 11|11, 11|1|1 and 1|1|1|1, where the bar represents start of the next row, where a(4) = A000041(L(4)) + A000041(L(3+1)) + A000041(L(2+2)) + A000041(L(2+1+1))+ A000041(L(1+1+1+1)) = A000041(1) + A000041(2) + A000041(2) + A000041(3) + A000041(4). By construction from sorted partitions, all the plane partitions are strictly decreasing along each row and each column. - R. J. Mathar, Aug 12 2008 Also the number of pairs of integer partitions, the first with sum n and the second with sum equal to the length of the first. - Gus Wiseman, Jul 19 2018 REFERENCES W. C. Yang, Derivatives of self-compositions of functions, preprint, 1997. LINKS Alois P. Heinz, Table of n, a(n) for n = 0..3000 (terms n = 501..959 from Vaclav Kotesovec) W. C. Yang, Derivatives are essentially integer partitions, Discrete Mathematics, 222(1-3), July 2000, 235-245. FORMULA If a(n,m) = number of terms in m-derivative of a function composed with itself n times, p(n,k) = number of partitions of n into k parts, then a(n,m) = Sum_{i=0..m} p(m,i)*a(n-1,i). EXAMPLE From Gus Wiseman, Jul 19 2018: (Start) Using the chain rule, we compute the second derivative of f(f(f(x))) to be the following sum of a(2) = 3 terms.   d^2/dx^2 f(f(f(x))) =   f'(f(x)) f'(f(f(x))) f''(x) +   f'(x)^2 f'(f(f(x))) f''(f(x)) +   f'(x)^2 f'(f(x))^2 f''(f(f(x))). (End) MAPLE A022811 := proc(n) local a, P, p, lp ; a := 0 ; P := combinat[partition](n) ; for p in P do lp := nops(p) ; a := a+combinat[numbpart](lp) ; od: RETURN(a) ; end: for n from 1 do print(n, A022811(n)) ; od: # R. J. Mathar, Aug 12 2008 MATHEMATICA a[n_] := Total[PartitionsP[Length[#]]& /@ IntegerPartitions[n]]; Table[an = a[n]; Print["a(", n, ") = ", an]; an, {n, 0, 80}] (* Jean-François Alcover, Apr 28 2017 *) Table[Length[1+D[f[f[f[x]]], {x, n}]]-1, {n, 10}] (* Gus Wiseman, Jul 19 2018 *) CROSSREFS Column k=3 of A022818. First column of A039805. A row or column of A081718. Cf. A008778, A022812, A022813, A022814, A022815, A022816, A022817, A024207, A024208, A024209, A024210, A131408. Sequence in context: A048134 A058397 A174369 * A295730 A323580 A002799 Adjacent sequences:  A022808 A022809 A022810 * A022812 A022813 A022814 KEYWORD nonn AUTHOR Winston C. Yang (yang(AT)math.wisc.edu) EXTENSIONS Typo corrected by Neven Juric, Mar 25 2013 STATUS approved

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Last modified February 22 18:05 EST 2019. Contains 320400 sequences. (Running on oeis4.)