A244207 - OEIS

A244207

Numbers 2m such that A020482(m) != A060308(m-1).

98, 122, 128, 148, 190, 208, 220, 250, 292, 302, 308, 326, 332, 346, 368, 398, 410, 418, 430, 458, 476, 488, 500, 518, 532, 538, 556, 586, 628, 640, 670, 692, 700, 710, 718, 728, 752, 770, 782, 788, 796, 806, 820, 838, 848, 854, 872, 878, 896, 902, 908, 920

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

OFFSET

1,1

COMMENTS

Usually the greatest p with p, q both prime, p + q = 2m, is equal to the largest prime <= 2m - 2, but not always. These are the exceptions, called "b numbers" ("b-Zahlen") by Nils Johan Pipping.

Almost all numbers are in the sequence. Only the beginning of the sequence is sparse. - Jens Kruse Andersen, Jul 12 2014

Let p be the smallest prime < 2n such that 2n-p is prime, and let r < 2n be the smallest odd nonprime > 1 such that 2n-r is prime; this sequence lists the numbers 2n for which r < p. - David James Sycamore, Jan 14 2019

LINKS

Jens Kruse Andersen, Table of n, a(n) for n = 1..10000

EXAMPLE

From David James Sycamore, Jan 14 2019: (Start)

98 is a term because (see 3rd comment above) p = 11 and r = 9.

128 is a term because then p = 19 and r = 15.

908 is a term because then p = 31 and r = 21. (End)

PROG

(PARI) for(m=2, 1000, p=precprime(2*m-2); if(!isprime(2*m-p), print1(2*m", "))) \\ Jens Kruse Andersen, Jul 12 2014

CROSSREFS

Cf. A020482, A060308. Equals twice A135145.

Sequence in context: A095589 A039491 A283555 * A344843 A109553 A268253