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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

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, 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.)