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A092250 Lesser of the greatest twin prime pair with n digits. 2

%I

%S 5,71,881,9929,99989,999959,9999971,99999587,999999191,9999999701,

%T 99999999761,999999999959,9999999998489,99999999999971,

%U 999999999997967,9999999999999641,99999999999998807,999999999999998927

%N Lesser of the greatest twin prime pair with n digits.

%C Sum of reciprocals = 0.215331408...

%C Also the numerator of the largest prime-over-prime fraction less than 1 that is the ratio of two primes both less than 10^n. - _Cino Hilliard_, Feb 13 2006 [edited by _Jon E. Schoenfield_, Dec 01 2019]

%H Robert G. Wilson v, <a href="/A092250/b092250.txt">Table of n, a(n) for n = 1..1250</a>

%t Array[Block[{k = 10^# - 3}, While[! AllTrue[{k, k + 2}, PrimeQ], k -= 2]; k] &, 18]

%o (PARI) lasttwpr(n) = { sr=0; for(m=0,n, c=0; forstep(x=10^(m+1)-1,10^m,-2, if(isprime(x)&& isprime(x-2),print1(x-2",");sr+=1./(x-2);break) ) ); print(); print(sr) }

%K base,nonn

%O 1,1

%A _Cino Hilliard_, Feb 17 2004

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Last modified February 26 05:51 EST 2020. Contains 332277 sequences. (Running on oeis4.)