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A225883
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a(n) = (-1)^n * (1 - 2^n).
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5
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0, 1, -3, 7, -15, 31, -63, 127, -255, 511, -1023, 2047, -4095, 8191, -16383, 32767, -65535, 131071, -262143, 524287, -1048575, 2097151, -4194303, 8388607, -16777215, 33554431, -67108863, 134217727, -268435455, 536870911, -1073741823, 2147483647, -4294967295
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OFFSET
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0,3
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COMMENTS
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A001047(n) = p(0) where p(x) is the unique degree-(n-1) polynomial such that p(k) = a(k) for k = 1, 2, ..., n.
a(n) = p(n) where p(x) is the unique degree-(n-1) polynomial such that p(k) = (-1)^k for k = 0, 1, ..., n-1.
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LINKS
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FORMULA
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G.f.: x / ((1 + x) * (1 + 2*x)) = 1 / (1+x) - 1 / (1 + 2*x).
E.g.f.: exp(-x) - exp(-2*x). a(n) = -2 * a(n-1) - (-1)^n if n>0.
a(n) = -(-1)^n * A000225(n). a(n) = -3 * a(n-1) - 2 * a(n-2) if n>1.
REVERT transform is A001003 omitting a(0)=0.
2^n = a(n+1)^2 - a(n) * a(n+2).
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EXAMPLE
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G.f. = x - 3*x^2 + 7*x^3 - 15*x^4 + 31*x^5 - 63*x^6 + 127*x^7 - 255*x^8 + 511*x^9 + ...
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MATHEMATICA
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a[ n_] := If[ n<0, 0, (-1)^n (1 - 2^n)];
LinearRecurrence[{-3, -2}, {0, 1}, 50] (* G. C. Greubel, Aug 09 2018 *)
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PROG
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(PARI) {a(n) = if( n<0, 0, (-1)^n * (1 - 2^n))};
(PARI) {a(n) = if( n<0, 0, polcoeff( x / ((1 + x) * (1 + 2*x)) + x * O(x^n), n))};
(Magma) [(-1)^n*(1 - 2^n): n in [0..50]]; // G. C. Greubel, Aug 09 2018
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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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