login
This site is supported by donations to The OEIS Foundation.

 

Logo

"Email this user" was broken Aug 14 to 9am Aug 16. If you sent someone a message in this period, please send it again.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A125524 Republican primes. Primes such that the right half of the prime is prime and the left half is not. 3

%I

%S 13,17,43,47,67,83,97,103,107,113,127,137,157,163,167,173,193,197,433,

%T 443,457,463,467,487,607,613,617,643,647,653,673,677,683,823,827,853,

%U 857,863,877,883,887,907,937,947,953,967,977,983,997,1013,1019,1031

%N Republican primes. Primes such that the right half of the prime is prime and the left 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.

%F Half n is the floor of the length of n divided by 2.

%e The right half of 13 is 3 which is prime. The left half is 1 which is not prime.

%e The right half of 113 is 3 which is prime. The left half is 1 which is not prime.

%o (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",") ) ) }

%Y Cf. A125523, 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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified August 20 01:30 EDT 2017. Contains 290821 sequences.