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A092250 Lesser of the greatest twin prime pair with n digits. 2
5, 71, 881, 9929, 99989, 999959, 9999971, 99999587, 999999191, 9999999701, 99999999761, 999999999959, 9999999998489, 99999999999971, 999999999997967, 9999999999999641, 99999999999998807, 999999999999998927 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Sum of reciprocals = 0.215331408...

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]

LINKS

Robert G. Wilson v, Table of n, a(n) for n = 1..1250

MATHEMATICA

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

PROG

(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) }

CROSSREFS

Sequence in context: A197668 A064752 A033507 * A193436 A193501 A133990

Adjacent sequences:  A092247 A092248 A092249 * A092251 A092252 A092253

KEYWORD

base,nonn

AUTHOR

Cino Hilliard, Feb 17 2004

STATUS

approved

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Last modified January 19 20:41 EST 2020. Contains 331066 sequences. (Running on oeis4.)