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A132458
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Let df(n,k) = Product_{i=0..k-1} (n-i) be the descending factorial and let P(m,n) = df(n-1,m-1)^2*(2*n-m)/((m-1)!*m!). Sequence gives P(4,n).
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6
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0, 0, 0, 1, 24, 200, 1000, 3675, 10976, 28224, 64800, 136125, 266200, 490776, 861224, 1449175, 2352000, 3699200, 5659776, 8450649, 12346200, 17689000, 24901800, 34500851, 47110624, 63480000, 84500000, 111223125, 144884376, 186924024, 239012200, 303075375
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,5
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COMMENTS
| P(m,n) is the number of n-step paths that start from (0,0) and reach (m,m) for the first time, where the steps are of the following 4 types: N=(x,y)->(x,y+1), E=(x,y)->(x+1,y), NE=(x,y)->(x+1,y+1), LOOP=(x,y)->(x,y).
For m = 1 through 8 we get respectively A005408, A000578, A108674, this sequence, A133317, A132464, A132465, A132466.
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LINKS
| Index to sequences with linear recurrences with constant coefficients, signature (8,-28,56,-70,56,-28,8,-1).
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FORMULA
| a(n) = (n-3)^2*(n-2)^3*(n-1)^2/72. G.f.: x^4*(1+16*x+36*x^2+16*x^3+x^4)/(1-x)^8. - Bruno Berselli, Dec 29 2010
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MAPLE
| df:=proc(n, k) mul(n-i, i=0..k-1); end; P:=proc(n, k) df(k-1, n-1)^2*(2*k-n)/((n-1)!*n!); end; [seq(P(4, n), n=1..50)];
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CROSSREFS
| Sequence in context: A126519 A198396 A048355 * A055857 A047659 A108671
Adjacent sequences: A132455 A132456 A132457 * A132459 A132460 A132461
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KEYWORD
| nonn
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AUTHOR
| Ottavio D'Antona (dantona(AT)dico.unimi.it), Oct 31 2007
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