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A185201 10^n - second largest prime less than 10^n.

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