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A116505 Number of distinct prime divisors of the concatenation of 1..n. 18

%I #19 Jul 07 2023 16:48:40

%S 0,2,2,2,3,3,2,4,3,3,6,4,3,3,3,3,4,5,6,6,8,6,4,5,4,6,5,5,4,7,3,5,6,2,

%T 7,5,4,4,6,8,5,7,4,4,9,7,5,7,6,9,3,3,4,9,5,4,6,4,4,6,3,7,4,9,6,8,3,7,

%U 7,6,5,5,3,9,5,4,5,6,6,7,4,7,6,3,5,7,6,5,9,8,6,6,7,5,6,5,2,9,5,9

%N Number of distinct prime divisors of the concatenation of 1..n.

%C Dario Alpern's factorization program was used for n > 43.

%H Dario Alpern, <a href="https://www.alpertron.com.ar/ECM.HTM">Factorization using the Elliptic Curve Method</a>

%e 123456 = 2*2*2*2*2*2*3*643, with distinct prime divisors 2, 3 and 643. Hence, a(6) = 3.

%t Table[PrimeNu[FromDigits[Flatten[IntegerDigits[Range[n]]]]], {n, 30}] (* _Jan Mangaldan_, Jul 07 2020 *)

%o (PARI) {a="";for(n=1,43,a=concat(a,n);print1(omega(eval(a)),", "))}

%Y Cf. A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138793.

%K nonn,base

%O 1,2

%A _Parthasarathy Nambi_, Mar 20 2006

%E Edited and extended by _Klaus Brockhaus_, Mar 29 2006

%E Terms 59-100 from _Sean A. Irvine_, Nov 04 2009

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Last modified March 29 08:59 EDT 2024. Contains 371268 sequences. (Running on oeis4.)