A124450 - OEIS

%I #14 Sep 17 2024 20:40:13

%S 5,47,491,4919,49877,499943,4999913,49999757,499999931,4999999937,

%T 49999999811,499999999769,4999999998431,49999999999619,

%U 499999999999769,4999999999998557,49999999999998887,499999999999999679

%N Lesser of a pair of not necessarily distinct closest primes that add up to 10^n.

%C a(n) is always an n digit number.

%C Note that if distinct primes are required, the only change is that a(1) = 3.

%H Hans Havermann, <a href="/A124450/b124450.txt">Table of n, a(n) for n = 1..50</a>

%F 10^n - a(n) is prime.

%e 10^1=5+5; 10^2=47+53; 10^3=491+509;

%e 10^4=4919+5081; 10^5=49877=50123; 10^6=499943+500057;

%e 10^7=4999913+5000087; 10^8=49999757+50000243;

%e 10^9=499999931+500000069;

%e 10^10=4999999937+5000000063; etc.

%t Table[ h =10^n/2; c=0; While[ PrimeQ[ h-c ]==False || PrimeQ[ h+c ]==False, c++ ]; h-c, {n, 1, 50} ] (from Hans Havermann, Nov 02 2006)

%Y Cf. A065577 = number of Goldbach partitions of 10^n.

%Y Cf. A124013.

%K nonn

%O 1,1

%A _Zak Seidov_, Nov 02 2006

%E Edited by _N. J. A. Sloane_ May 15 2008 at the suggestion of _R. J. Mathar_.