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 A119819 a(n) equals the coefficient of x^(n-1) in the (n-1)-th iteration of g.f. A(x) for n>1, with a(1)=1. 5
 1, 1, 2, 12, 138, 2370, 54190, 1553258, 53883088, 2211883428, 105760271082, 5819880201432, 364979361177134, 25865387272507770, 2056021496464455000, 182094050389241652004, 17861355920109599058260, 1929874166854161381238676, 228564755268775651632722308, 29540844190975459101114949972 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Here the zeroth iteration of A(x) equals x, the first iteration is itself, the 2nd iteration of A(x) = A(A(x)), etc. LINKS EXAMPLE The coefficients in the n-th iteration of g.f. A(x) begin: n=1: [1, 1,  2,   12,   138,   2370,   54190,  1553258,   53883088, ...]; n=2: [1, 2,  6,   35,   370,   6000,  132344,  3704032,  126318024, ...]; n=3: [1, 3, 12,   75,   758,  11612,  245746,  6688885,  223699238, ...]; n=4: [1, 4, 20,  138,  1388,  20322,  411708, 10854152,  354952262, ...]; n=5: [1, 5, 30,  230,  2370,  33760,  656414, 16711414,  532707614, ...]; n=6: [1, 6, 42,  357,  3838,  54190, 1018484, 25016120,  775036254, ...]; n=7: [1, 7, 56,  525,  5950,  84630, 1553258, 36874397, 1107956996, ...]; n=8: [1, 8, 72,  740,  8888, 128972, 2337800, 53883088, 1568966580, ...]; n=9: [1, 9, 90, 1008, 12858, 192102, 3476622, 78308058, 2211883428, ...]; ... where the diagonal of coefficients equals this sequence shift left 1 place. ... More explicitly, the successive iterations of g.f. A(x) begin: A(x) = (1)x + x^2 + 2x^3 + 12x^4 + 138x^5 + 2370x^6 + 54190x^7 +... A(A(x)) = x + (2)x^2 + 6x^3 + 35x^4 + 370x^5 + 6000x^6 + 132344x^7 +... A(A(A(x))) = x + 3x^2 + (12)x^3 + 75x^4 + 758x^5 + 11612x^6 +... A(A(A(A(x)))) = x + 4x^2 + 20x^3 + (138)x^4 + 1388x^5 + 20322x^6 +... A(A(A(A(A(x))))) = x + 5x^2 + 30x^3 + 230x^4 + (2370)x^5 + 33760x^6+... A(A(A(A(A(A(x)))))) = x + 6x^2 +42x^3 +357x^4 +3838x^5 + (54190)x^6+... ... PROG (PARI) {a(n)=local(F=x+x^2+sum(m=3, n-1, a(m)*x^m), G=x+x*O(x^n)); if(n<1, 0, if(n<=2, 1, for(i=1, n-1, G=subst(F, x, G)); return(polcoeff(G, n-1, x))))} for(n=1, 30, print1(a(n), ", ")) CROSSREFS Cf. A112317, A119820, A119821, A119815, A119817. Sequence in context: A117513 A297078 A185522 * A093543 A287885 A091144 Adjacent sequences:  A119816 A119817 A119818 * A119820 A119821 A119822 KEYWORD nonn AUTHOR Paul D. Hanna, May 31 2006 STATUS approved

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Last modified January 16 15:31 EST 2019. Contains 319195 sequences. (Running on oeis4.)