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A245511 Smallest m such that the largest odd number < n^m is not prime. 6
1, 1, 2, 3, 2, 3, 2, 4, 1, 1, 2, 3, 2, 4, 1, 1, 2, 4, 2, 3, 1, 1, 2, 2, 1, 1, 1, 1, 2, 2, 2, 3, 1, 1, 1, 1, 2, 2, 1, 1, 2, 3, 2, 2, 1, 1, 2, 3, 1, 1, 1, 1, 2, 3, 1, 1, 1, 1, 2, 3, 2, 3, 1, 1, 1, 1, 2, 3, 1, 1, 2, 2, 2, 3, 1, 1, 1, 1, 2, 2, 1, 1, 2, 2, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,3

COMMENTS

The locution "largest odd number < n^m" means n^m-1 for even n and n^m-2 for odd n.

The record breakers of this sequence are a(2)=1, a(4)=2, a(5)=3, a(9)=4, a(279)=5, a(15331)=6, a(1685775)=7. No higher value was found up to 5500000 (see also A245512). It is not clear whether a(n) is bounded.

LINKS

Stanislav Sykora, Table of n, a(n) for n = 2..10000

EXAMPLE

a(2)=1 because 2^1-1 is 1, which is not a prime.

a(5)=3 because the numbers 5^k-2, for k=1,2,3,.., are 3,23,123,... and the first nonprime among them corresponds to k=3.

MATHEMATICA

f[n_] := Block[{m = 1, d = If[ OddQ@ n, 2, 1]}, While[t = n^m - d; EvenQ@ t || PrimeQ@ t, m++]; m]; Array[f, 105, 2] (* Robert G. Wilson v, Aug 04 2014 *)

PROG

(PARI) avector(nmax)={my(n, k, d=2, v=vector(nmax)); for(n=2, #v+1, d=3-d; k=1; while(1, if(!isprime(n^k-d), v[n-1]=k; break, k++)); ); return(v); }

a=avector(10000)  \\ For nmax=6000000 runs out of 1GB memory

CROSSREFS

Cf. A245509, A245510, A245512, A245513, A245514.

Sequence in context: A103509 A252941 A069898 * A259940 A228829 A007978

Adjacent sequences:  A245508 A245509 A245510 * A245512 A245513 A245514

KEYWORD

nonn

AUTHOR

Stanislav Sykora, Jul 24 2014

STATUS

approved

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Last modified August 18 22:05 EDT 2017. Contains 290768 sequences.