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A089026 a(n) = n if n is a prime, otherwise a(n) = 1. 17
1, 2, 3, 1, 5, 1, 7, 1, 1, 1, 11, 1, 13, 1, 1, 1, 17, 1, 19, 1, 1, 1, 23, 1, 1, 1, 1, 1, 29, 1, 31, 1, 1, 1, 1, 1, 37, 1, 1, 1, 41, 1, 43, 1, 1, 1, 47, 1, 1, 1, 1, 1, 53, 1, 1, 1, 1, 1, 59, 1, 61, 1, 1, 1, 1, 1, 67, 1, 1, 1, 71, 1, 73, 1, 1, 1, 1, 1, 79, 1, 1, 1, 83, 1, 1, 1, 1, 1, 89, 1, 1, 1, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

Paulo Ribenboim, The little book of big primes, Springer 1991, p. 106.

L. Tesler, "Factorials and Primes", Math. Bulletin of the Bronx H.S. of Science (1961), 5-10. [From Larry Tesler (tesler(AT)pobox.com), Nov 08 2010]

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..5000

FORMULA

From Peter Luschny, Nov 29 2003: (Start)

a(n) = denominator(n! * Sum_{m=0..n} (-1)^m*m!*Stirling2(n+1, m+1)/(m+1)).

a(n) = denominator(n! * Sum_{m=0..n} (-1)^m*m!*Stirling2(n, m)/(m+1)). (End)

From Alexander Adamchuk, May 20 2006: (Start)

a(n) = numerator((n/2)/(n-1)!) + floor(2/n) - 2*floor(1/n).

a(n) = A090585(n-1) = A000217(n-1)/A069268(n-1) for n>2. (End)

a(n) = gcd(n,(n-1)!+1). - Jaume Oliver Lafont, Jul 17 2008, Jan 23 2009

a(n) = n*((n-1)!^2 mod n)+(((n+1)-(n-1)!^2) mod n)+(C(2*(n-1),n-1) mod 2), with n>=1. - Paolo P. Lava, Feb 17 2009

a(1) = 1, a(2) = 2, then a(n) = 1 or a(n) = n = prime(m) = (Product q+k, k = 1 ... 2*floor(n/2+1)-q) / (Product prime(i)^(Sum (floor((n+1)/(prime(i)^w)) - floor(q/(prime(i)^w)) ), w = 1 ... floor(log[base prime(i)] n+1) ), i = 2 ... m-1) where q = prime(m-1). -  Larry Tesler (tesler(AT)pobox.com), Nov 08 2010

a(n) = (n!*HarmonicNumber(n) mod n)+1, n != 4. - Gary Detlefs, Dec 03 2011

a(n) = denominator of (n!)/n^(3/2). - Arkadiusz Wesolowski, Dec 04 2011

a(n) = A034386(n+1)/A034386(n). - Eric Desbiaux, May 10 2013

a(n) = n^A010051(n). - Wesley Ivan Hurt, Jun 16 2013

a(n) = A014963(n)^(-A008683(n)). - Mats Granvik, Jul 02 2016

Conjecture: for n > 3, a(n) = gcd(n, A007406(n-1)). - Thomas Ordowski, Aug 02 2019

EXAMPLE

From Larry Tesler (tesler(AT)pobox.com), Nov 08 2010: (Start)

a(9) = (8*9*10)/(2^((5+2+1)-(3+1+0))*3^((3+1)-(2+0))*5^((2)-(1))*7^((1)-(1))) = 1 [composite].

a(10) = (8*9*10)/(2^((5+2+1)-(3+1+0))*3^((3+1)-(2+0))*5^((2)-(1))*7^((1)-(1))) = 1 [composite].

a(11) = (8*9*10*11*12)/(2^((6+3+1)-(3+1+0))*3^((4+1)-(2+0))*5^((2)-(1))*7^((1)-(1))) = 11 [prime]. (End)

MATHEMATICA

digits=200; a=Table[If[PrimePi[n]-PrimePi[n-1]>0, n, 1], {n, 1, digits}]; Table[Numerator[(n/2)/(n-1)! ] + Floor[2/n] - 2*Floor[1/n], {n, 1, 200}] (* Alexander Adamchuk, May 20 2006 *)

Range@ 120 /. k_ /; CompositeQ@ k -> 1 (* or *)

Table[n Boole@ PrimeQ@ n, {n, 120}] /. 0 -> 1 (* or *)

Table[If[PrimeQ@ n, n, 1], {n, 120}] (* Michael De Vlieger, Jul 02 2016 *)

PROG

(Sage)

def A089026(n):

    if n == 4: return 1

    f = factorial(n-1)

    return (f + 1) - n*(f//n)

[A089026(n) for n in (1..96)]   # Peter Luschny, Oct 16 2013

(MAGMA) [IsPrime(n) select n else 1: n in [1..96]]; // Marius A. Burtea, Aug 02 2019

CROSSREFS

Differs from A080305 at n=30.

Cf. A090585, A000217, A069268, A090586, A007619, A135683, A007406.

Sequence in context: A139764 A227643 A249386 * A080305 A220137 A053815

Adjacent sequences:  A089023 A089024 A089025 * A089027 A089028 A089029

KEYWORD

nonn

AUTHOR

Roger L. Bagula, Nov 12 2003

STATUS

approved

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Last modified July 5 17:04 EDT 2020. Contains 335473 sequences. (Running on oeis4.)