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A284721 Smallest odd prime that is relatively prime to 2n+1. 3
3, 5, 3, 3, 5, 3, 3, 7, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 7, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 7, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 11, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 7, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 7, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 7, 3, 3, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

More than the usual number of terms are shown in order to distinguish this from A239278.

a(n) = smallest odd prime missing from rad(2*n+1).

If rad(2m+1) = rad(2n+1), a(m) = a(n) (cf. A007947). - Bob Selcoe, Apr 04 2017

LINKS

Table of n, a(n) for n=0..100.

FORMULA

a(n) = 3 unless n == 1 (mod 3).

For all n >= 2, a(n) < 3*log(2*n+1). Also, for all n >= 1, a(n) <5*log(2*n+1). [Upper bound corrected by N. J. A. Sloane, Apr 15 2017. Thanks to Bob Selcoe for pointing out that the old bound failed at n=1.]

PROG

(PARI) a(n) = my(p=3); while(gcd(2*n+1, p) != 1, p=nextprime(p+1)); p; \\ Michel Marcus, Apr 04 2017

CROSSREFS

Similar to but different from A239278.

Cf. A007947, A284722, A284723.

Sequence in context: A092553 A112755 A239278 * A126659 A246917 A102294

Adjacent sequences:  A284718 A284719 A284720 * A284722 A284723 A284724

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Apr 04 2017

STATUS

approved

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Last modified June 26 10:12 EDT 2019. Contains 324375 sequences. (Running on oeis4.)