|
%I #16 May 26 2019 04:39:19
%S 29,31,59,71,79,211,229,239,241,251,269,271,281,311,331,349,359,379,
%T 389,509,521,541,569,571,599,701,709,719,739,751,761,769,1109,1151,
%U 1163,1181,1187,1193,1301,1321,1327,1381,1399,1709,1721,1733,1777,1787,1901
%N Democratic primes: primes such that the left half of the prime is prime and the right half is not.
%C 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.
%H Tony Padilla and Brady Haran, <a href="https://www.youtube.com/watch?v=zk_Q9y_LNzg">The Most Evil Number</a>, Numberphile video (2018)
%e The left half of 29 is 2, which is prime. The right half is 9, which is not prime.
%e The left half of 211 is 2, which is prime. The right half is 1, which is not prime.
%o (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",") ) ) }
%Y Cf. A125524, A125525.
%K base,easy,nonn
%O 2,1
%A _Cino Hilliard_, Jan 22 2007
|
