A175734 Largest n-digit number with 3 divisors.

9, 49, 961, 9409, 97969, 994009, 9840769, 99460729, 999002449, 9998200081, 99996985729, 999966000289, 9999995824729, 99999820000081, 999997874844049, 9999997800000121, 99999977853408361, 999999874000003969, 9999999828172082689, 99999999340000001089

Offset: 1

Author

Jaroslav Krizek, Aug 24 2010

Keywords

• nonn
• base

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. - Harvey P. Dale, Aug 18 2011 [with thanks to Ant King and Christopher Hunt Gribble]

Links

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

Crossrefs

Cf. A131581, A132153, A136582, A175733.

Programs

Magma

A175734 := func; // Jason Kimberley, Aug 18 2011

Mathematica

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

PARI

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

Formula

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

Extensions

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