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 A098482 Expansion of 1/sqrt((1-x)^2-4*x^4). 4
 1, 1, 1, 1, 3, 7, 13, 21, 37, 73, 147, 283, 531, 1007, 1953, 3817, 7423, 14371, 27877, 54333, 106189, 207585, 405743, 793719, 1554889, 3049525, 5984803, 11751067, 23086695, 45388291, 89289765, 175746797, 346077153, 681795925, 1343790319 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS From Joerg Arndt, Jul 01 2011: (Start) Empirical: Number of lattice paths from (0,0) to (n,n) using steps (4,0), (0,4), (1,1). It appears that 1/sqrt((1-x)^2-4*x^s) is the g.f. for lattice paths from (0,0) to (n,n) using steps (s,0), (0,s), (1,1). Empirical: Number of lattice paths from (0,0) to (n,n) using steps (3,1), (1,3), (1,1).  (End) 1/sqrt((1-x)^2-4*r*x^4) expands to sum(k=0..floor(n/2), binomial(n-2*k,k)*binomial(n-3*k,k)*r^k ). LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..200 Steffen Eger, On the Number of Many-to-Many Alignments of N Sequences, arXiv:1511.00622 [math.CO], 2015. FORMULA a(n) = sum(k=0..floor(n/2), binomial(n-2*k, k)*binomial(n-3*k, k) ). Recurrence: n*a(n) = (2*n-1)*a(n-1) - (n-1)*a(n-2) + 4*(n-2)*a(n-4). - Vaclav Kotesovec, Jun 23 2014 a(n) ~ 2^(n+1/2) / sqrt(3*Pi*n). - Vaclav Kotesovec, Jun 23 2014 EXAMPLE From Joerg Arndt, Jul 01 2011: (Start) The triangle of lattice paths from (0,0) to (n,k) using steps (3,1), (1,3), (1,1) begins 1; 0, 1; 0, 0, 1; 0, 1, 0, 1; 0, 0, 2, 0, 3; 0, 0, 0, 3, 0, 7; 0, 0, 1, 0, 4, 0, 13; 0, 0, 0, 3, 0, 8, 0, 21; 0, 0, 0, 0, 6, 0, 18, 0, 37; 0, 0, 0, 1, 0, 10, 0, 37, 0, 73; The triangle of lattice paths from (0,0) to (n,k) using steps (4,0), (0,4), (1,1) begins 1; 0, 1; 0, 0, 1; 0, 0, 0, 1; 1, 0, 0, 0, 3; 0, 2, 0, 0, 0, 7; 0, 0, 3, 0, 0, 0, 13; 0, 0, 0, 4, 0, 0, 0, 21; 1, 0, 0, 0, 8, 0, 0, 0, 37; 0, 3, 0, 0, 0, 18, 0, 0, 0, 73; The diagonals of both appear to be this sequence.  (End) MAPLE seq(add(binomial(n-3*k, k)*binomial(n-2*k, k), k=0..floor(n/3)), n=0..34); # Zerinvary Lajos, Apr 03 2007 MATHEMATICA CoefficientList[Series[1/Sqrt[(1-x)^2-4*x^4], {x, 0, 20}], x] (* Vaclav Kotesovec, Jun 23 2014 *) PROG (PARI) /* as lattice paths, assuming the first comment is true */ /* same as in A092566 but use either of */ steps=[[4, 0], [0, 4], [1, 1]]; steps=[[3, 1], [1, 3], [1, 1]]; /* Joerg Arndt, Jul 01 2011 */ CROSSREFS Cf. A098479, A098483, A098484. Sequence in context: A098575 A138035 A032606 * A147432 A018367 A146369 Adjacent sequences:  A098479 A098480 A098481 * A098483 A098484 A098485 KEYWORD easy,nonn AUTHOR Paul Barry, Sep 10 2004 STATUS approved

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Last modified October 18 04:57 EDT 2019. Contains 328145 sequences. (Running on oeis4.)