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A126130 a(n) = (n+1)^n - n!. 4
1, 7, 58, 601, 7656, 116929, 2092112, 43006401, 999637120, 25933795801, 742968453888, 23297606120881, 793708546233344, 29192838847099425, 1152920196932478976, 48661170952876980481, 2185911204051268435968 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Fit a polynomial f of degree n-1 to the first n n-th powers of positive integers. Then a(n) = f(n+1). It is not necessary to actually determine the polynomial f; a(n) can be found by considering differences.

a(n-1) is also the number of labeled rooted trees on n objects that are not increasing; i.e., at least one node has a label smaller than its parent's label. a(n) is the number of partial functions on n labeled objects that are not permutations. - Franklin T. Adams-Watters, Dec 25 2006

Equal to the number of partial functions [n]->[n] which are not permutations (equivalently, the number of non-surjective partial functions [n]->[n]); i.e. equal to the cardinality of the complement PT_n\S_n where PT_n and S_n denote the partial transformation semigroup and symmetric group on [n]. - James East, May 03 2007

LINKS

Seiichi Manyama, Table of n, a(n) for n = 1..386

FORMULA

The polynomial f is equal to Sum_{k=1}^n -s(n+1,k) x^{k-1}, where the s(n,k) are the Stirling numbers of the first kind (A008275). - Franklin T. Adams-Watters, Dec 25 2006

E.g.f.: -1/(1 - x) - LambertW(-x)/(x*(1 + LambertW(-x))), where LambertW() is the Lambert W-function. - Ilya Gutkovskiy, Aug 22 2018

EXAMPLE

The quadratic that fits (1,1), (2,8) and (3,27) is f(n) = 6n^2-11n+6. Then a(3) = f(4) = 58.

MATHEMATICA

Table[(n+1)^n-n!, {n, 30 }] (* Harvey P. Dale, Jun 06 2015 *)

PROG

(PARI) vector(18, n, (n+1)^n-n!)

(MAGMA) [(n+1)^n - n!: n in [1..20]]; // Altug Alkan, Mar 19 2018

CROSSREFS

Cf. A000169, A000142.

Sequence in context: A163048 A318233 A192940 * A323254 A123766 A005332

Adjacent sequences:  A126127 A126128 A126129 * A126131 A126132 A126133

KEYWORD

easy,nonn

AUTHOR

Nick Hobson (nickh(AT)qbyte.org), Dec 18 2006

STATUS

approved

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Last modified June 17 15:07 EDT 2019. Contains 324185 sequences. (Running on oeis4.)