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
Links
- 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.
Programs
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Maple
q:= n-> is(numtheory[bigomega](parse(cat($1..n)))=3): select(q, [$1..35])[]; # Alois P. Heinz, Apr 10 2021
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Mathematica
Select[Range[100], PrimeOmega@FromDigits@Flatten@IntegerDigits@Range@# == 3 &] (* Robert Price, Oct 11 2019 *)
Formula
A046460(a(n)) = 3.
Extensions
a(10)-a(11) from Sean A. Irvine, Apr 10 2021