A046462 Numbers k such that the concatenation of numbers from 1 to k is the product of 3 primes (not necessarily distinct).

2, 5, 10, 13, 14, 15, 31, 51, 61, 67, 73

Offset

1,1

Links

Table of n, a(n) for n=1..11.

Patrick De Geest, Normal Smarandache Concatenated Numbers, Prime factors from 1 up to n

M. Fleuren, Factors and primes of Smarandache sequences.

Carlos Rivera, Puzzle 8. Primes by Listing, The Prime Puzzles & Problems Connection.

Formula

A046460(a(n)) = 3.

Maple

q:= n-> is(numtheory[bigomega](parse(cat($1..n)))=3):
select(q, [$1..35])[];  # Alois P. Heinz, Apr 10 2021

Mathematica

Select[Range[100],
PrimeOmega@FromDigits@Flatten@IntegerDigits@Range@# == 3 &] (* Robert Price, Oct 11 2019 *)

Crossrefs

Cf. A001222, A046460.

Sequence in context: A090050 A274454 A192539 * A188025 A387350 A187949

Adjacent sequences: A046459 A046460 A046461 * A046463 A046464 A046465

Keyword

nonn,hard,base

Author

Patrick De Geest, Aug 15 1998

Extensions

a(10)-a(11) from Sean A. Irvine, Apr 10 2021

Status

approved