This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A125523 Democratic primes: primes such that the left half of the prime is prime and the right half is not. 3
 29, 31, 59, 71, 79, 211, 229, 239, 241, 251, 269, 271, 281, 311, 331, 349, 359, 379, 389, 509, 521, 541, 569, 571, 599, 701, 709, 719, 739, 751, 761, 769, 1109, 1151, 1163, 1181, 1187, 1193, 1301, 1321, 1327, 1381, 1399, 1709, 1721, 1733, 1777, 1787, 1901 (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 left half of 29 is 2, which is prime. The right half is 9, which is not prime. The left half of 211 is 2, which is prime. The right half is 1, which is not prime. PROG (PARI) /* Political primes, democratic case. */ dem(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. A125524, A125525. Sequence in context: A273547 A132240 A132243 * A156976 A157681 A288615 Adjacent sequences:  A125520 A125521 A125522 * A125524 A125525 A125526 KEYWORD base,easy,nonn AUTHOR Cino Hilliard, Jan 22 2007 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified June 18 07:44 EDT 2019. Contains 324203 sequences. (Running on oeis4.)