A242993 Least k such that R = (2^k*Q-Q-1)/(Q+1-2^k) is prime, where Q = A000668(n) is the n-th Mersenne prime, or 0 if no such k exists.

0, 2, 4, 4, 11, 13, 16, 16, 57, 78, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0

Offset

1,2

Comments

Kravitz has shown that 2^(k-1)*Q*R is a primitive weird number (cf. A002975) when Q > 2^k =: M+1 and R = (M*Q-1)/(Q-M) = M + (M^2-1)/(Q-M) both are prime. R cannot be an integer unless Q < M(M+1) which yields k > p/2 for Mersenne primes Q = 2^p-1. [Edited by M. F. Hasler, Nov 11 2018]

Sequence A242025 lists all primes R obtained in that way. Sequence A242998 gives the number of (k,R) for each Q in A000668. Sequence A242998 lists the primes p which give rise to a solution, with multiplicity, and A243003 lists the corresponding values of k. See the "main entry" A242025 for more information. - M. F. Hasler, Nov 11 2018

Links

Table of n, a(n) for n=1..37.

S. Kravitz, A search for large weird numbers. J. Recreational Math. 9(1976), 82-85 (1977). Zbl 0365.10003

Example

For n = 2, Q = A000668(2) = 7, k = 2 yields the prime R = (2^k*Q-Q-1)/(Q+1-2^k) = 20/4 = 5 and the (smallest possible) weird number 2^(k-1)*Q*R = 2*7*5 = 70.
For n = 9, Q = A000668(9) = 2^61-1, k = 57 yields the prime R = 2^57-1 + (2^57-2)/(2^4-1) and the 53-digit primitive weird number 2^56*Q*R = 25541592347764814106588251084767772206406532903993344.
For n = 10, Q = A000668(10) = 2^89-1, k = 78 yields the prime R = 2^78-1 + (2^78-2)/(2^11-1) and the 74-digit primitive weird number 2^77*Q*R = 28283363272427014026275183563912621451964887156507346985599492888375328768.

Mathematica

A000043 = {2, 3, 5, 7, 13, 17, 19, 31, 61, 89, 107, 127, 521, 607,
   1279, 2203, 2281, 3217, 4253, 4423, 9689, 9941, 11213, 19937,
   21701, 23209, 44497, 86243, 110503, 132049, 216091, 756839, 859433,
   1257787, 1398269, 2976221, 3021377, 6972593, 13466917, 20996011,
   24036583, 25964951, 30402457, 32582657, 37156667, 42643801,
   43112609};
lst = {};
For[i = 1, i <= 25, i++,
  p = A000043[[i]];
  kc = 0;
  For[k = 1, k < p, k++,
   r = 2^k - 1 + (2^k - 2)/(2^(p - k) - 1);
   If[! IntegerQ[r], Continue[]];
   If[PrimeQ[r], kc = k; Break[]]];
  AppendTo[lst, kc]];
lst (* Robert Price, Sep 05 2019 *)

Prog

(PARI) a(n)={p=A000043[n]; for(k=p\2+1, p-1, Mod(2, 2^(p-k)-1)^k==2 && ispseudoprime(2^k-1+(2^k-2)/(2^(p-k)-1)) && return(k))}

Crossrefs

Cf. A242025 (primes R obtained in that way), A242998 (number of such k values for given n), A242999 (p=A000043(n) listed A242998(n) times), A243003 (all values of k).

Cf. A258882 (weird numbers of the form 2^k*p*q).

Sequence in context: A199825 A121049 A056415 * A366045 A263382 A286714

Adjacent sequences: A242990 A242991 A242992 * A242994 A242995 A242996

Keyword

nonn,more,hard

Author

M. F. Hasler, Aug 17 2014

Extensions

Definition corrected by Jens Kruse Andersen, Aug 18 2014

a(28)-a(37) from Robert Price, Sep 05 2019

Status

approved