A180927 Largest n-digit number that is divisible by exactly 3 primes (counted with multiplicity).

8, 99, 994, 9994, 99997, 999994, 9999994, 99999994, 999999998, 9999999995, 99999999998, 999999999998, 9999999999998, 99999999999998, 999999999999995, 9999999999999998, 99999999999999998, 999999999999999987, 9999999999999999999

Offset

1,1

Comments

This is to 3 and A014612, as 2 and A098450 (largest n-digit semiprime), and as 1 and A003618 (largest n-digit prime). Largest n-digit triprime. Largest n-digit 3-almost prime.

Links

Table of n, a(n) for n=1..19.

Example

a(1) = 8 because 8 = 2^3 is the largest (only) 1-digit number that is divisible by exactly 3 primes (counted with multiplicity).
a(2) = 99 because 99 = 3^2 * 11 is the largest 2-digit number (of 21) that is divisible by exactly 3 primes (counted with multiplicity).
a(3) = 994 because 994 = 2 * 7 * 71 is the largest 3-digit number that is divisible by exactly 3 primes (counted with multiplicity).

Mathematica

lndn3[n_]:=Module[{k=10^n-1}, While[PrimeOmega[k]!=3, k--]; k]; Array[ lndn3, 20] (* Harvey P. Dale, Jul 25 2019 *)

Prog

(PARI) A180927(n)=forstep(n=10^n-1, 10^(n-1), -1, bigomega(n)==3&return(n)) \\ M. F. Hasler, Jan 23 2011

Crossrefs

Cf. A003618, A014612, A098450, A180922.

Sequence in context: A299513 A299313 A300114 * A091801 A050919 A341965

Adjacent sequences: A180924 A180925 A180926 * A180928 A180929 A180930

Keyword

nonn,base,easy

Author

Jonathan Vos Post, Jan 23 2011

Status

approved