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 A125524 Republican primes: primes such that the right half of the prime is prime and the left half is not. 3
 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; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS 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. LINKS Tony Padilla and Brady Haran, The Most Evil Number, Numberphile video (2018) EXAMPLE 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. PROG (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", ") ) ) } CROSSREFS Cf. A125523, A125525. Sequence in context: A279392 A239901 A068497 * A156553 A248474 A140533 Adjacent sequences:  A125521 A125522 A125523 * A125525 A125526 A125527 KEYWORD base,easy,nonn AUTHOR Cino Hilliard, Jan 22 2007 STATUS approved

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Last modified October 19 13:01 EDT 2019. Contains 328222 sequences. (Running on oeis4.)