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A169998
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a(0)=1, a(1)=1; thereafter a(n) = -a(n-1) - 2*a(n-2).
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5
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1, 1, -3, 1, 5, -7, -3, 17, -11, -23, 45, 1, -91, 89, 93, -271, 85, 457, -627, -287, 1541, -967, -2115, 4049, 181, -8279, 7917, 8641, -24475, 7193, 41757, -56143, -27371, 139657, -84915, -194399, 364229, 24569, -753027, 703889, 802165, -2209943, 605613, 3814273, -5025499, -2603047
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OFFSET
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0,3
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COMMENTS
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Cassels, following Nagell, shows that a(n) = +- 1 only for n = 1, 2, 3, 5, 13.
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REFERENCES
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J. W. S. Cassels, Local Fields, Cambridge, 1986, see p. 67.
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LINKS
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FORMULA
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G.f.: ( 1+2*x ) / ( 1+x+2*x^2 ). - R. J. Mathar, Jul 14 2011
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MAPLE
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f:=proc(n) option remember; if n <= 1 then 1 else -f(n-1)-2*f(n-2); fi; end;
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MATHEMATICA
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PROG
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CROSSREFS
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KEYWORD
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sign,easy
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AUTHOR
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STATUS
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approved
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