A215339 -id:A215339

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A306634

Numbers k such that A215339(6*k+2) = 0.

+20
0

0, 2755452, 4570452, 123725995972

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Numbers k such that b(k) is an integer, where b(n) = Sum_{k=0..n} (2*n+k)!/((2*n-2*k)!*(3*k+1)!) (= A001608(6*n+2)/(6*n+2)).

LINKS

Table of n, a(n) for n=1..4.

Don Zagier, Problems posed at the St Andrews Colloquium, 1996, See "Fifth Day: Sequences and recursions, problem 2".

EXAMPLE

n | b(n)

--+--------------------------

0 | 1 (= 2/2)

1 | 5/4 (= 10/8)

2 | 51/14

3 | 277/20

4 | 1497/26

5 | 4045/16 (= 8090/32)

6 | 43721/38

7 | 118141/22 (= 236282/44)

8 | 638471/25 (= 1276942/50)

9 | 6900995/56

CROSSREFS

Cf. A001608, A013998, A215339.

KEYWORD

nonn,more

AUTHOR

Seiichi Manyama, Mar 02 2019

STATUS

approved

A013998

Unrestricted Perrin pseudoprimes.

+10
16

271441, 904631, 16532714, 24658561, 27422714, 27664033, 46672291, 102690901, 130944133, 196075949, 214038533, 517697641, 545670533, 801123451, 855073301, 903136901, 970355431, 1091327579, 1133818561, 1235188597, 1389675541, 1502682721, 2059739221, 2304156469, 2976407809, 3273820903

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

"The column Mathematical Recreations by Ian Stewart in the June 1996 issue of Scientific American discusses the Perrin sequence [A001608] A(n) defined by A(0)=3, A(1)=0, A(2)=2, A(n+1)=A(n-1)+A(n-2). Motivated by a theorem of E. Lucas: If n is prime it divides A(n) exactly, the question whether primality of n follows from n divides A(n) exactly was formulated 1899. So far, they say, nobody has found a composite n that divides A(n). Such a number would be called a Perrin pseudoprime. The article quotes an experiment by Steven Arno of the Supercomputing Research Center in Bowie, Md., where a lower bound of 15 digits for the size of the smallest Perrin pseudoprime was obtained in 1991. On Jul 3rd, 1996, I was able to find the two smallest Perrin pseudoprimes:" [Holzbaur] - Robert G. Wilson v, Nov 30 2001

In the "Feedback" section of his column for November 1996, Ian Stewart mentions that Jeffrey Shallit (Waterloo) had written to him saying that he had found the Perrin pseudoprimes 271441 and 904631 in 1982.

There are 765 Perrin pseudoprimes which are also Carmichael numbers less than 2^64. - Dana Jacobsen, May 10 2015

There are 101994 Perrin pseudoprimes which are also Fermat pseudoprimes to base 2 less than 2^64. - Dana Jacobsen, May 10 2015

Difference in the number of unlabeled maximal independent sets of an a(n)-cycle graph A127687(a(n)) times a(n), from value of Perrin(a(n)) such that Perrin(a(n)) mod a(n) = Sum_{d|a(n)} (Perrin(d)*Phi(a(n)/d)) mod a(n) [d<a(n)]==0 (mod a(n)); Perrin = A001608, Phi = A000010. - Richard Turk, Aug 11 2015

There are two known Perrin pseudoprimes that are squares: a(1)=271441=521^2 and a(76)=36366108601=190699^2. In A173656 it is claimed that there are no others < (10^9)^2. - Hugo Pfoertner, Sep 01 2017

LINKS

Dana Jacobsen, Table of n, a(n) for n = 1..1702 (first 658 terms from Robert Harley)

William W. Adams and Daniel Shanks, Strong primality tests that are not sufficient, Math. Comp. 39 (1982), 255-300.

Robert Dougherty-Bliss, Experimental Methods in Number Theory and Combinatorics, Ph. D. Dissertation, Rutgers Univ. (2024). See p. 34.

Jon Grantham, There are infinitely many Perrin pseudoprimes, Journal of Number Theory Volume 130, Issue 5, May 2010, Pages 1117-1128.

Christian Holzbaur, Perrin pseudoprimes [Original link broke many years ago. This is a cached copy from the WayBack machine, dated Apr 24 2006]

Dana Jacobsen, Pseudoprime Statistics, Tables, and Data

Holger Stephan, Perrin pseudoprimes up to 10^16 with factorization. [Note: this is not a complete list of Perrin pseudoprimes in the range, Dana Jacobsen, May 10 2015]

Holger Stephan, Perrin pseudoprimes up to 10^16 with factorization. [Note: this is not a complete list of Perrin pseudoprimes in the range, Dana Jacobsen, May 10 2015] [Cached copy, with permission]

Holger Stephan, Millions of Perrin pseudoprimes including a few giants, arXiv:2002.03756 [math.NA], 2020.

Ian Stewart, Tales of a Neglected Number. Mathematical Recreations, Scientific American, 6 (1996), 92-93.

Ian Stewart, Tales of a Neglected Number, Mathematical Recreations, Scientific American, Vol. 274, No. 6 (1996), pp. 102-103.

Eric Weisstein's World of Mathematics, Perrin Pseudoprime.

Index entries for sequences related to pseudoprimes

PROG

(PARI)

N=10^10;

default(primelimit, N);

M = [0, 1, 0; 0, 0, 1; 1, 1, 0];

a(n)=lift( trace( Mod(M, n)^n ) ); /* A215339(n) */

{ for (n=1, N,

if ( isprime(n), next() );

if ( a(n)==0, print1(n, ", "); );

); }

/* Joerg Arndt, Aug 16 2012 */

(Perl)

use ntheory ":all";

forcomposites { say if is_perrin_pseudoprime($_) } 1e10;

# Dana Jacobsen, May 10 2015

CROSSREFS

Cf. A000010, A001608, A018187, A173656.

KEYWORD

nonn

AUTHOR

R. K. Guy

EXTENSIONS

More terms from alipson(AT)cix.compulink.co.uk (Andrew Lipson)

STATUS

approved

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