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Search: id:A125524
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| A125524 |
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Republican primes. Primes such that the right half of the prime is prime and the left half is not. |
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+0
2
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| 13, 17, 43, 47, 67, 83, 97, 103, 107, 113, 127, 137, 157, 163, 167, 173, 193, 197, 433, 443, 457, 463, 467, 487, 607, 613, 617, 643, 647, 653, 673, 677, 683, 823, 827, 853, 857, 863, 877, 883, 887, 907, 937, 947, 953, 967, 977, 983, 997, 1013, 1019, 1031
(list; graph; listen)
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OFFSET
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2,1
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COMMENT
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If the length of n is odd then the central number is not used in the calculation. So neither the left half nor the right half will contain the central digit. If the length of n is even, then all numbers are used.
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FORMULA
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Half n is the floor of the length of n divided by 2.
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EXAMPLE
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The right half of 13 is 3 which is prime. The left half is 1 which is not prime.
The right half of 113 is 3 which is prime. The left half is 1 which is not prime.
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PROGRAM
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(PARI) \Political primes, republican case. rep(n) = { local(x, ln, y, lp, rp); forprime(x=2, n, y=Str(x); if(x > 9, ln=floor(length(y)/2), ln=1); lp = eval(left(y, ln)); rp = eval(right(y, ln)); if(!isprime(lp)&& isprime(rp), print1(x", ") ) ) }
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CROSSREFS
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Sequence in context: A043113 A043893 A068497 this_sequence A156553 A140533 A076789
Adjacent sequences: A125521 A125522 A125523 this_sequence A125525 A125526 A125527
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KEYWORD
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base,easy,nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)hotmail.com), Jan 22 2007
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