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 A046462 Numbers k such that the concatenation of numbers from 1 to k is the product of 3 primes (not necessarily distinct). 2
 2, 5, 10, 13, 14, 15, 31, 51, 61, 67, 73 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 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, 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 A187949 A180161 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

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Last modified September 21 06:57 EDT 2021. Contains 347596 sequences. (Running on oeis4.)