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A090519 Smallest prime p such that floor((10^n)/p) is prime, or 0 if no such number exists. 5

%I #13 Jul 31 2023 10:08:08

%S 2,13,23,13,89,19,7,47,67,13,17,157,17,313,107,409,151,773,149,409,

%T 109,13,29,211,7,19,149,431,859,43,109,167,277,13,2293,173,907,107,

%U 1087,617,449,1013,73,1249,743,109,233,499,191,479

%N Smallest prime p such that floor((10^n)/p) is prime, or 0 if no such number exists.

%C Conjecture: No term is zero. Subsidiary Sequence: Number of primes in floor((10^n)/p), p is a prime. a(1) = 3, the primes are 10/2, floor(10/3) and 10/5.

%H Robert Israel, <a href="/A090519/b090519.txt">Table of n, a(n) for n = 1..1800</a>

%e a(5) = 89, as floor((10^5)/89) = 1123 is the largest such prime.

%p f:= proc(n) local t,p;

%p t:= 10^n;

%p p:= 1;

%p while p < t/2 do

%p p:= nextprime(p);

%p if isprime(floor(t/p)) then return p fi

%p od;

%p 0

%p end proc:

%p map(f, [$1..50]); # _Robert Israel_, Jul 30 2023

%t <<NumberTheory`; Do[k = 2; While[ !PrimeQ[Floor[10^n / k]], k = NextPrime[k]]; Print[k], {n, 1, 50}] (* _Ryan Propper_, Jun 19 2005 *)

%Y Cf. A090517, A090518, A090520.

%K base,nonn

%O 1,1

%A _Amarnath Murthy_, Dec 07 2003

%E Corrected and extended by _Ryan Propper_, Jun 19 2005

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Last modified March 29 06:44 EDT 2024. Contains 371265 sequences. (Running on oeis4.)