

A182648


a(n) is the largest ndigit number with exactly 4 divisors.


2



8, 95, 998, 9998, 99998, 999997, 9999998, 99999997, 999999991, 9999999997, 99999999997, 999999999997, 9999999999989, 99999999999997, 999999999999998, 9999999999999994, 99999999999999989, 999999999999999993, 9999999999999999991, 99999999999999999983
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OFFSET

1,1


COMMENTS

a(n) is the largest ndigit number of the form p^3 or p^1*q^1, (p, q = distinct primes).


LINKS

Michael S. Branicky, Table of n, a(n) for n = 1..62


FORMULA

A000005(a(n)) = 4.


MATHEMATICA

Table[k=10^n1; While[DivisorSigma[0, k] != 4, k]; k, {n, 10}]


PROG

(Python)
from sympy import divisors
def a(n):
k = 10**n  1
divs = 1
while divs != 4:
k = 1
divs = 0
for d in divisors(k, generator=True):
divs += 1
if divs > 4: break
return k
print([a(n) for n in range(1, 21)]) # Michael S. Branicky, Jun 10 2021


CROSSREFS

Subsequence of A030513.
Cf. A174322, A098450.
Sequence in context: A298659 A299611 A099298 * A003775 A262737 A299747
Adjacent sequences: A182645 A182646 A182647 * A182649 A182650 A182651


KEYWORD

nonn,base


AUTHOR

Jaroslav Krizek, Nov 27 2010


EXTENSIONS

a(19) and beyond from Michael S. Branicky, Jun 10 2021


STATUS

approved



