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A175734 Largest n-digit number with 3 divisors. 7
9, 49, 961, 9409, 97969, 994009, 9840769, 99460729, 999002449, 9998200081, 99996985729, 999966000289, 9999995824729, 99999820000081, 999997874844049, 9999997800000121, 99999977853408361, 999999874000003969, 9999999828172082689, 99999999340000001089 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Because every number with exactly three divisors must be the square of a prime, the last digit of each term in this sequence is either 1 or 9. [From Harvey P. Dale, Aug 18 2011, with thanks to Ant King and Chris Gribble]

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..1000

FORMULA

a(n) = A132153(n)^2.

MATHEMATICA

Join[{9}, Table[NextPrime[Sqrt[10^n-1], -1]^2, {n, 2, 20}]](* Harvey P. Dale, Aug 18 2011 *)

PROG

(MAGMA) A175734 := func<n|PreviousPrime(Isqrt(10^n)+1)^2>; /* From Jason Kimberley, Aug 18 2011 */

(PARI) A175734(n)=precprime(sqrtint(10^n))^2  \\ M. F. Hasler, Aug 18 2011

CROSSREFS

Cf. A131581, A132153, A136582, A175733.

Sequence in context: A160603 A115326 A069666 * A174744 A133049 A188140

Adjacent sequences:  A175731 A175732 A175733 * A175735 A175736 A175737

KEYWORD

nonn,base

AUTHOR

Jaroslav Krizek, Aug 24 2010

EXTENSIONS

a(14)-a(20) from Harvey P. Dale, Aug 18 2011

STATUS

approved

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Last modified November 20 08:12 EST 2017. Contains 294962 sequences.