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Search: id:A125525
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| A125525 |
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Centrist primes. Primes such that both the right half and the left half of the prime is prime. |
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2
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| 2, 3, 5, 7, 23, 37, 53, 73, 223, 227, 233, 257, 263, 277, 283, 293, 307, 313, 317, 337, 347, 353, 367, 373, 383, 397, 503, 523, 547, 557, 563, 577, 587, 593, 727, 733, 743, 757, 773, 787, 797, 1103, 1117, 1123, 1129, 1153, 1171, 1303, 1307, 1319, 1361, 1367
(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 > 9 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. My guess is these numbers are infinite.
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FORMULA
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Half n > 9 is the floor of the length of n divided by 2. For n <= 9 half n is 1.
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EXAMPLE
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The right half of 23 is 2 which is prime. The left half is 3 which is also prime so 23 is a centrist prime.
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PROGRAM
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(PARI) \Political primes, Centrist 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: A113611 A038925 A074491 this_sequence A019546 A096148 A124674
Adjacent sequences: A125522 A125523 A125524 this_sequence A125526 A125527 A125528
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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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