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 A068811 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. 5
 3, 5, 7, 11, 17, 29, 41, 47, 53, 59, 71, 83, 89, 97, 113, 137, 173, 179, 191, 227, 239, 257, 281, 317, 347, 353, 359, 383, 401, 431, 443, 479, 491, 509, 521, 557, 569, 599, 617, 641, 647, 653, 683, 719, 743, 761, 773, 809, 821, 827, 863, 887, 911, 929, 941 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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 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 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 EXAMPLE 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. 641 is a term as 641 and 1000-641 = 359 are primes. MATHEMATICA Select[Prime[Range[160]], PrimeQ[10^(Floor[Log[10, # ]] + 1) - # ] &] (* Stefan Steinerberger, Jun 15 2007 *) PROG (PARI) is_A068811(p)= isprime(10^#Str(p)-p) & isprime(p) \\ M. F. Hasler, May 01 2012 (PARI) for(d=1, 4, forprime(p=10^(d-1), 10^d, if(isprime(10^d-p), print1(p", ")))) \\ Charles R Greathouse IV, May 01 2012 (Sage) [p for p in prime_range(100) if is_prime(10^p.ndigits()-p)] # Giuseppe Coppoletta, Jul 24 2016 CROSSREFS Sequence in context: A241896 A076186 A092564 * A088083 A116457 A037155 Adjacent sequences:  A068808 A068809 A068810 * A068812 A068813 A068814 KEYWORD easy,nonn,base AUTHOR Amarnath Murthy, Mar 07 2002 EXTENSIONS Corrected by Jason Earls, May 25 2002 Edited by N. J. A. Sloane, Sep 18 2008 at the suggestion of R. J. Mathar STATUS approved

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Last modified June 23 02:02 EDT 2021. Contains 345395 sequences. (Running on oeis4.)