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%I #21 Jul 19 2024 21:39:18
%S 5,11,9,33,11,21,27,29,71,57,53,39,137,29,53,83,23,33,57,27,113,71,53,
%T 303,321,249,107,261,53,17,81,119,47,513,237,179,123,123,173,27,203,
%U 137,119,77,119,147,83,47,183,161,333,339,611,579
%N 10^n - second largest prime less than 10^n.
%D D. E. Knuth, The Art of Computer Programming, Second Edition, Vol. 2, Seminumerical Algorithms, Chapter 4.5.4 Factoring into Primes, Table 1, Page 390, Addison-Wesley, Reading, MA, 1981.
%H Robert Israel, <a href="/A185201/b185201.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = 10^n - precprime(precprime(10^n)-1)
%e a(1) = 5 because precprime(10) = 7, and precprime(6) = 5.
%e From _M. F. Hasler_, Jul 19 2024: (Start)
%e Further examples: (where pp = prevprime = A151799)
%e n | pp(pp(10^n)) | a(n)
%e ----+-----------------+------
%e 1 | 5 | 5
%e 2 | 89 | 11
%e 3 | 991 | 9
%e 4 | 9967 | 33
%e 5 | 99989 | 11
%e 6 | 999979 | 21
%e 7 | 9999973 | 27
%e 8 | 99999971 | 29
%e 9 | 999999929 | 71
%e 10 | 9999999943 | 57
%e 11 | 99999999947 | 53
%e 12 | 999999999961 | 39
%e 13 | 9999999999863 | 137
%e 14 | 99999999999971 | 29
%e 15 | 999999999999947 | 53
%e (End)
%p seq(10^n - prevprime(prevprime(10^n)),n=1..100); # _Robert Israel_, May 28 2017
%t Table[10^n - NextPrime[10^n, -2], {n,1,50}] (* _G. C. Greubel_, Jun 24 2017 *)
%o (PARI) apply( {A185201(n)=10^n-precprime(precprime(10^n)-1)}, [1..66]) \\ _M. F. Hasler_, Jul 19 2024
%Y Cf. A033874.
%Y Cf. A003618 (largest prime < 10^n), A151799 (prevprime function).
%K nonn
%O 1,1
%A _Washington Bomfim_, Jan 24 2012