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 A047970 Antidiagonal sums of nexus numbers (A047969). 20
 1, 2, 5, 14, 43, 144, 523, 2048, 8597, 38486, 182905, 919146, 4866871, 27068420, 157693007, 959873708, 6091057009, 40213034874, 275699950381, 1959625294310, 14418124498211, 109655727901592, 860946822538675, 6969830450679864, 58114638923638573 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS From Lara Pudwell, Oct 23 2008 (Start): A permutation p avoids a pattern q if it has no subsequence that is order-isomorphic to q. For example, p avoids the pattern 132 if it has no subsequence abc with a 1, b(m) <= max{b(m-1) + 1, max{b(i) | 1 <= i <= m - 1}}. This sequence begins 1, 2, 5, 14, 43, 144, 523, 2048, 8597, 38486. The term 144 counts the length 6 sequence 1, 2, 3, 1, 1, 3, for instance. Contrast with the families of sequences discussed in Franklin T. Adams-Watters's comment in A005425. - Rick L. Shepherd, Jan 01 2015 a(n-1) for n>=1 is the number of length-n restricted growth strings (RGS)  [s(0), s(1), ..., s(n-1)] with s(0)=0 and s(k) <= the number of fixed points in the prefix, see example. - Joerg Arndt, Mar 08 2015 Number of sequences (e(1), ..., e(n+1)), 0 <= e(i) < i, such that there is no triple i < j < k with e(i) != e(j) = e(k). [Martinez and Savage, 2.15] - Eric M. Schmidt, Jul 17 2017 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..300 G. E. Andrews, The Theory of Partitions, 1976, page 242 table of Gaussian polynomials. D. Callan, The number of bar(31)542-avoiding permutations, arXiv:1111.3088 Megan A. Martinez and Carla D. Savage, Patterns in Inversion Sequences II: Inversion Sequences Avoiding Triples of Relations, arXiv:1609.08106 [math.CO], 2016. Lara Pudwell, Enumeration Schemes for Pattern-Avoiding Words and Permutations, Ph. D. Dissertation, Math. Dept., Rutgers University, May 2008. Eric Weisstein's World of Mathematics, Nexus Number FORMULA Formal o.g.f.: (1 - x)*( sum {n >= 0} x^n/(1 - (n + 2)*x) ). - Peter Bala, Jul 09 2014 O.g.f.: Sum_{n>=0} (n+1)! * x^n/(1-x)^(n+1) / Product_{k=1..n+1} (1 + k*x). - Paul D. Hanna, Jul 20 2014 O.g.f.: Sum_{n>=0} x^n / ( (1 - n*x) * (1 - (n+1)*x) ). - Paul D. Hanna, Jul 22 2014 EXAMPLE a(3) = 1 + 5 + 7 + 1 = 14. From Paul D. Hanna, Jul 22 2014:  (Start) G.f. A(x) = 1 + 2*x + 5*x^2 + 14*x^3 + 43*x^4 + 144*x^5 + 523*x^6 + 2048*x^7 + ... where we have the series identity: A(x) = (1-x)*( 1/(1-2*x) + x/(1-3*x) + x^2/(1-4*x) + x^3/(1-5*x) + x^4/(1-6*x) + x^5/(1-7*x) + x^6/(1-8*x) +... ) is equal to A(x) = 1/(1-x) + x/((1-x)*(1-2*x)) + x^2/((1-2*x)*(1-3*x)) + x^3/((1-3*x)*(1-4*x)) + x^4/((1-4*x)*(1-5*x)) + x^5/((1-5*x)*(1-6*x)) + x^6/((1-6*x)*(1-7*x)) +... and also equals A(x) = 1/((1-x)*(1+x)) + 2!*x/((1-x)^2*(1+x)*(1+2*x)) + 3!*x^2/((1-x)^3*(1+x)*(1+2*x)*(1+3*x)) + 4!*x^3/((1-x)^4*(1+x)*(1+2*x)*(1+3*x)*(1+4*x)) +... (End) From Joerg Arndt, Mar 08 2015: (Start) There are a(4-1)=14 length-4 RGS as in the comment (dots denote zeros): 01:  [ . . . . ] 02:  [ . . . 1 ] 03:  [ . . 1 . ] 04:  [ . . 1 1 ] 05:  [ . 1 . . ] 06:  [ . 1 . 1 ] 07:  [ . 1 . 2 ] 08:  [ . 1 1 . ] 09:  [ . 1 1 1 ] 10:  [ . 1 1 2 ] 11:  [ . 1 2 . ] 12:  [ . 1 2 1 ] 13:  [ . 1 2 2 ] 14:  [ . 1 2 3 ] (End) MAPLE T := proc(n, k) option remember; local j;     if k=n then 1   elif k>n then 0   else (k+1)*T(n-1, k) + add(T(n-1, j), j=k..n)     fi end: A047970 := n -> T(n, 0); seq(A047970(n), n=0..24); # Peter Luschny, May 14 2014 MATHEMATICA a[ n_] := SeriesCoefficient[ ((1 - x) Sum[ x^k / (1 - (k + 2) x), {k, 0, n}]), {x, 0, n}]; (* Michael Somos, Jul 09 2014 *) PROG (Sage) def A074664():     T = []; n = 0     while True:         T.append(1)         yield T[0]         for k in (0..n):             T[k] = (k+1)*T[k] + add(T[j] for j in (k..n))         n += 1 a = A074664() [a.next() for n in range(25)] # Peter Luschny, May 13 2014 (PARI) /* From o.g.f. (Paul D. Hanna, Jul 20 2014) */ {a(n)=polcoeff( sum(m=0, n, (m+1)!*x^m/(1-x)^(m+1)/prod(k=1, m+1, 1+k*x +x*O(x^n))), n)} for(n=0, 25, print1(a(n), ", ")) (PARI) /* From o.g.f. (Paul D. Hanna, Jul 22 2014) */ {a(n)=polcoeff( sum(m=0, n, x^m/((1-m*x)*(1-(m+1)*x +x*O(x^n)))), n)} for(n=0, 25, print1(a(n), ", ")) CROSSREFS Partial sums are in A026898, A003101. First differences A112532. Cf. A112531. Sequence in context: A014327 A173437 A137550 * A160701 A137551 A148333 Adjacent sequences:  A047967 A047968 A047969 * A047971 A047972 A047973 KEYWORD nonn AUTHOR Alford Arnold, Dec 11 1999 STATUS approved

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