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 A225721 Starting with x = n, the number of iterations of x := 2x - 1 until x is prime, or -1 if no prime exists. 1
 -1, 0, 0, 1, 0, 1, 0, 2, 1, 1, 0, 1, 0, 2, 1, 1, 0, 3, 0, 6, 1, 1, 0, 1, 2, 2, 1, 2, 0, 1, 0, 8, 3, 1, 2, 1, 0, 2, 5, 1, 0, 1, 0, 2, 1, 2, 0, 583, 1, 2, 1, 1, 0, 1, 1, 4, 1, 2, 0, 5, 0, 4, 7, 1, 2, 1, 0, 2, 1, 1, 0, 3, 0, 2, 1, 1, 4, 3, 0, 2, 3, 1, 0, 1, 2, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,8 COMMENTS This appears to be a shifted variant of A040076. - R. J. Mathar, May 28 2013 If n is prime, then a(n) = 0. If the sequence never reaches a prime number (for n = 1) or the prime number has more than 1000 digits, -1 is used instead. There are 22 such numbers for n < 10000. LINKS Christian N. K. Anderson, Table of n, a(n) for n = 1..10000 EXAMPLE For a(20), the trajectory is 20->39->77->153->305->609->1217, a prime number. That required 6 steps, so a(20)=6. PROG (R) y=as.bigz(rep(0, 500)); ys=rep(0, 500); for(i in 1:500) { n=as.bigz(i); k=0;     while(isprime(n)==0 & ndig(n)<1000 & k<5000) { k=k+1; n=2*n-1 }     if(ndig(n)>=1000 | k>=5000) { ys[i]=-1; y[i]=-1;     } else {ys[i]=k; y[i]=n; } } CROSSREFS Cf. A050921 (primes obtained). Cf. A040081, A038699, A050412, A052333, A046069 (related to the Riesel problem). Cf. A000668, A000043, A065341 (Mersenne primes), A000079 (powers of 2). Cf. A007770 (happy numbers), A031177 (unhappy numbers). Cf. A037274 (home primes), A037271 (steps), A037272, A037272. Sequence in context: A328620 A257510 A305445 * A040076 A019269 A204459 Adjacent sequences:  A225718 A225719 A225720 * A225722 A225723 A225724 KEYWORD sign AUTHOR Kevin L. Schwartz and Christian N. K. Anderson, May 13 2013 STATUS approved

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Last modified December 9 22:27 EST 2019. Contains 329880 sequences. (Running on oeis4.)