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A182648 a(n) is the largest n-digit 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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) is the largest n-digit 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^n-1; 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

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Last modified December 2 17:59 EST 2021. Contains 349445 sequences. (Running on oeis4.)