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

%I

%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

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 October 14 15:03 EDT 2019. Contains 328019 sequences. (Running on oeis4.)