%I
%S 3,5,7,11,17,29,41,47,53,59,71,83,89,97,113,137,173,179,191,227,239,
%T 257,281,317,347,353,359,383,401,431,443,479,491,509,521,557,569,599,
%U 617,641,647,653,683,719,743,761,773,809,821,827,863,887,911,929,941
%N Numbers n such that n and its 10's complement are both primes, i.e., n and 10^k  n where k is the number of digits in n, are primes.
%C In other words, primes p such that the difference between the smallest power of 10 exceeding p and p is prime.  _Zak Seidov_, Feb 27 2004
%C The only twin prime pairs in the sequence are (3,5) and (5,7). This is easily seen by mod 6 congruences using 10^k = 4 (mod 6).  _Giuseppe Coppoletta_, Jul 24 2016
%H Vincenzo Librandi, <a href="/A068811/b068811.txt">Table of n, a(n) for n = 1..1000</a>
%e 47 is a prime; the smallest power of 10 exceeding 47 is 100 and 100  47 = 53 is prime. Therefore 47 is in the sequence.
%e 641 is a term as 641 and 1000641 = 359 are primes.
%t Select[Prime[Range[160]], PrimeQ[10^(Floor[Log[10, # ]] + 1)  # ] &] (* _Stefan Steinerberger_, Jun 15 2007 *)
%o (PARI) is_A068811(p)= isprime(10^#Str(p)p) & isprime(p) \\ _M. F. Hasler_, May 01 2012
%o (PARI) for(d=1, 4, forprime(p=10^(d1), 10^d, if(isprime(10^dp), print1(p", ")))) \\ _Charles R Greathouse IV_, May 01 2012
%o (Sage) [p for p in prime_range(100) if is_prime(10^p.ndigits()p)] # _Giuseppe Coppoletta_, Jul 24 2016
%K easy,nonn,base
%O 1,1
%A _Amarnath Murthy_, Mar 07 2002
%E Corrected by _Jason Earls_, May 25 2002
%E Edited by _N. J. A. Sloane_, Sep 18 2008 at the suggestion of _R. J. Mathar_
