A175733 a(n) is the smallest n-digit number with 3 divisors.

4, 25, 121, 1369, 10201, 100489, 1018081, 10004569, 100140049, 1000267129, 10000600009, 100008370081, 1000006000009, 10000033772089, 100000380000361, 1000000025191729, 10000001400000049, 100000000621806289, 1000000014000000049, 10000000055856073561

Offset: 1

Author

Jaroslav Krizek, Aug 24 2010

Keywords

• nonn
• base

Crossrefs

Cf. A131581, A132153, A136582, A175734.

Programs

Mathematica

Table[Prime[1 + PrimePi[Sqrt[10^n]]]^2, {n, 0, 25}] (* T. D. Noe, Aug 19 2011 *)

Python

from math import sqrt
from sympy import nextprime
def a(n): return nextprime(int(sqrt(10**n+1)))**2
print([a(n) for n in range(20)]) # Michael S. Branicky, Apr 25 2021

Formula

a(n) = A131581(n-1)^2.