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A132153 Largest prime <= square root of 10^n. 7
3, 7, 31, 97, 313, 997, 3137, 9973, 31607, 99991, 316223, 999983, 3162277, 9999991, 31622743, 99999989, 316227731, 999999937, 3162277633, 9999999967, 31622776589, 99999999977, 316227766003, 999999999989, 3162277660153, 9999999999971, 31622776601657 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

To check if an n+1 digit number is prime, u(n) is the largest prime which one needs to check is not a factor of the n+1 th digit number. For example to check a general four digit number is not prime, we need to test its divisibility by all the primes up to and including 97.

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..2000

FORMULA

a(n)=A000040(A122121(n)). a(2n)=A003618(n). - R. J. Mathar, Nov 06 2007 [Corrected by Jaroslav Krizek, Jul 12 2010]

a(n) = sqrt(A175734(n)). [From Jaroslav Krizek, Aug 24 2010]

PROG

(PARI) a(n)=precprime(sqrtint(10^n)) \\ Charles R Greathouse IV, Aug 18 2011

CROSSREFS

Cf. A017934, A131581, A136582, A175733, A175734.

Sequence in context: A244114 A072881 A257924 * A002357 A173062 A306831

Adjacent sequences:  A132150 A132151 A132152 * A132154 A132155 A132156

KEYWORD

nonn

AUTHOR

Anthony C Robin, Nov 01 2007

EXTENSIONS

More terms from N. J. A. Sloane, Jan 05 2008

STATUS

approved

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Last modified October 16 16:34 EDT 2019. Contains 328101 sequences. (Running on oeis4.)