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A339579 a(n) = least nonnegative integer k such that n*2^k - 1 is composite. 8
4, 3, 5, 2, 0, 4, 0, 1, 0, 0, 0, 3, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 2, 0, 0, 0, 0, 0, 2, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 3, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjectured to grow without limit.

A063377 is an essentially identical sequence, although with a slightly different definition, different initial terms, and different offset.

REFERENCES

Carl Pomerance, Problem 81:21 (= 321), in R. K. Guy problem list.

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..65537

R. K. Guy, editor, Western Number Theory Problems, 1985-12-21 & 23, Typescript, Jul 13 1986, Dept. of Math. and Stat., Univ. Calgary, 11 pages. Annotated scan of pages 1, 3, 7, 9, with permission.

FORMULA

For n >= 3, a(n) = A063377(n-1).

PROG

(PARI) A339579(n) = for(k=0, oo, my(t=(n*(2^k))-1); if((t>1)&&!isprime(t), return(k))); \\ Antti Karttunen, Dec 24 2020

CROSSREFS

See A339580 for records.

Cf. A063377, A063378, A339581.

Sequence in context: A274260 A011397 A352692 * A081665 A131911 A254037

Adjacent sequences:  A339576 A339577 A339578 * A339580 A339581 A339582

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Dec 24 2020

STATUS

approved

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Last modified August 17 23:13 EDT 2022. Contains 356204 sequences. (Running on oeis4.)