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A005565
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Number of walks on square lattice.
(Formerly M5087)
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3
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20, 75, 189, 392, 720, 1215, 1925, 2904, 4212, 5915, 8085, 10800, 14144, 18207, 23085, 28880, 35700, 43659, 52877, 63480, 75600, 89375, 104949, 122472, 142100, 163995, 188325, 215264, 244992, 277695, 313565, 352800, 395604, 442187, 492765, 547560, 606800
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OFFSET
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0,1
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = 1/4*(n^4+14n^3+69n^2+136n+80). G.f.: (20-25x+14x^2-3x^3)/(1-x)^5. - Ralf Stephan, Apr 23 2004
a(n) = binomial(n+4,2)^2 - binomial(n+4,1)^2. - Gary Detlefs, Nov 22 2011
Using two consecutive triangular numbers t(n) and t(n+1), starting at n=3, compute the determinant of a 2 X 2 matrix with the first row t(n), t(n+1) and the second row t(n+1), 2*t(n+1). This gives (n+1)^2*(n-2)*(n+2)/4 = a(n-3). - J. M. Bergot, May 17 2012
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MAPLE
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A005565:=(-20+25*z-14*z**2+3*z**3)/(z-1)**5; # conjectured by Simon Plouffe in his 1992 dissertation
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MATHEMATICA
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CoefficientList[Series[(20-25x+14x^2-3x^3)/(1-x)^5, {x, 0, 40}], x] (* Vincenzo Librandi, May 24 2012 *)
LinearRecurrence[{5, -10, 10, -5, 1}, {20, 75, 189, 392, 720}, 40] (* Harvey P. Dale, Dec 04 2020 *)
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PROG
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(Magma) [1/4*(n^4+14*n^3+69*n^2+136*n+80): n in [0..40]]; // Vincenzo Librandi, May 24 2012
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CROSSREFS
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KEYWORD
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nonn,walk,easy
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AUTHOR
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STATUS
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approved
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