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A225721
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Starting with x = n, the number of iterations of x := 2x - 1 until x is prime, or -1 if no prime exists.
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1
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-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
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OFFSET
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1,8
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COMMENTS
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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.
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LINKS
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Christian N. K. Anderson, Table of n, a(n) for n = 1..10000
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EXAMPLE
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For a(20), the trajectory is 20->39->77->153->305->609->1217, a prime number. That required 6 steps, so a(20)=6.
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PROG
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(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; }
}
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CROSSREFS
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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
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KEYWORD
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sign
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AUTHOR
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Kevin L. Schwartz and Christian N. K. Anderson, May 13 2013
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STATUS
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approved
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