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A181983
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a(n) = (-1)^(n+1) * n.
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26
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0, 1, -2, 3, -4, 5, -6, 7, -8, 9, -10, 11, -12, 13, -14, 15, -16, 17, -18, 19, -20, 21, -22, 23, -24, 25, -26, 27, -28, 29, -30, 31, -32, 33, -34, 35, -36, 37, -38, 39, -40, 41, -42, 43, -44, 45, -46, 47, -48, 49, -50, 51, -52, 53, -54, 55, -56, 57, -58, 59
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OFFSET
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0,3
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COMMENTS
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This is the Lucas U(-2,1) sequence. - R. J. Mathar, Jan 08 2013
For n>0, a(n) is also the determinant of the symmetric n X n matrix M defined by M(i,j) = max(i,j) for 1 <= i,j <= n. - Enrique Pérez Herrero, Jan 14 2013
The sums of the terms of this sequence is the divergent series 1 - 2 + 3 - 4 + ... . Euler summed it to 1/4 which was one of the first examples of summing divergent series. - Michael Somos, Jun 05 2013
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LINKS
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FORMULA
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G.f.: x / (1 + x)^2.
E.g.f.: x / exp(x).
a(n) = -a(-n) = -(-1)^n * A001477(n) for all n in Z.
a(n+1) = p(n+1) where p(x) is the unique degree-n polynomial such that p(k) = Bernoulli(k) for k = 0, 1, ..., n.
A001787(n) = p(0) where p(x) is the unique degree-n polynomial such that p(k) = a(k) for k = 1, ..., n+1. - Michael Somos, Jun 05 2013
Euler transform of length 2 sequence [-2, 2].
Series reversion of g.f. is A000108(n) (Catalan numbers) with a(0)=0.
Multiplicative with a(2^e) = -2^e, and a(p^e) = p^e for an odd prime p.
Dirichlet g.f.: zeta(s-1) * (1-2^(2-s)). (End)
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EXAMPLE
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G.f. = x - 2*x^2 + 3*x^3 - 4*x^4 + 5*x^5 - 6*x^6 + 7*x^7 - 8*x^8 + 9*x^9 + ...
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MAPLE
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MATHEMATICA
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a[ n_] := -(-1)^n n;
a[ n_] := Sign[n] SeriesCoefficient[ x / (1 + x)^2, {x, 0, Abs @n}];
a[ n_] := Sign[n] (Abs @n)! SeriesCoefficient[ x / Exp[ x], {x, 0, Abs @n}];
CoefficientList[Series[x/(1+x)^2, {x, 0, 60}], x] (* or *) LinearRecurrence[{-2, -1}, {0, 1}, 60] (* or *) Table[If[OddQ[n], n, -n], {n, 0, 60}] (* Harvey P. Dale, Apr 22 2022 *)
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PROG
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(PARI) {a(n) = -(-1)^n * n};
(Haskell)
a181983 = negate . a038608
a181983_list = [0, 1] ++ map negate
(zipWith (+) a181983_list (map (* 2) $ tail a181983_list))
(Magma) [(-1)^(n+1)*n: n in [0..30]]; // G. C. Greubel, Aug 11 2018
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CROSSREFS
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KEYWORD
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sign,mult,easy
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AUTHOR
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STATUS
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approved
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