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 A078993 Starting at the chess position shown, a(n) is the number of ways Black can make n consecutive moves, followed by a checkmate in one move by White. 1
 0, 0, 0, 0, 0, 2, 5, 8, 28, 24, 108, 66, 357, 176, 1088, 464, 3160, 1218, 8901, 3192, 24564, 8360, 66836, 21890, 180037, 57312, 481464, 150048, 1280736, 392834, 3393509, 1028456, 8965324, 2692536, 23633532, 7049154, 62197413, 18454928, 163482992, 48315632, 429300136 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 COMMENTS Starting position: White queen at g8, king at h1; Black pawn at h7, king at h6. Black may not move into check. REFERENCES Problem composed by N. D. Elkies. LINKS R. P. Stanley, Extremal [Chess] Problems FORMULA G.f.: sum(a(n)*x^n, n=0..infinity) = x^5*(2+5*x-4*x^2-2*x^3)/((1-x^2)*(1-2*x^2)*(1-3*x^2+x^4)). a(2*n) = 3 - 2^(n+2) + F(2*n+3) for n>0 and a(2*n+1) = 2*(F(2*n-1)-1) with F(n) the Fibonacci numbers. EXAMPLE For n = 5 we have the move orders: (1): 1.Kh5 2.Kh4 3.Kh3 4.h5 5.h4; (2): 1.Kh5 2.Kh4 3.h5 4.Kh3 5.h4; both followed by Qg2# and a(5) = 2. For n = 6 we have the move orders: (1): 1.Kh5 2.Kh4 3.Kh3 4.h6 5.h5 6.h4; (2): 1.Kh5 2.Kh4 3.h6 4.h5 5.Kh3 6.h4; (3): 1.Kh5 2.Kh4 3.h6 4:Kh3 5.h5 6.h4; (4): 1.Kh5 2.h6 3.Kh4 4.Kh3 5.h5 6.h4; (5): 1.Kh5 2.h6 3.Kh4 4.h5 5.Kh3 6.h4; all followed by Qg2# and a(6) = 5. CROSSREFS Cf. A000045 (Fibonacci), A027941 (Fibonacci(2*n+1)-1). Sequence in context: A093365 A209865 A128600 * A066846 A140275 A025533 Adjacent sequences:  A078990 A078991 A078992 * A078994 A078995 A078996 KEYWORD nonn AUTHOR N. J. A. Sloane, Jan 18 2003 EXTENSIONS Formula corrected, examples, formulae and crossrefs added and edited by Johannes W. Meijer, Feb 06 2010 and Feb 8 2010. STATUS approved

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