A115165 - OEIS

%I #15 Sep 08 2022 08:45:23

%S 3,11,13,19,21,23,25,27,35,37,39,45,47,49,51,53,55,57,73,75,81,87,93,

%T 95,97,99,105,107,117,123,131,135,139,143,145,147,155,159,161,163,165,

%U 169,177,181,193,195,201,207,213,215,217,221,225,229,239,243,249,259,265

%N Odd numbers k such that k-1 and k+1 have the same number of distinct prime divisors.

%H Amiram Eldar, <a href="/A115165/b115165.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[1, 301, 2], PrimeNu[#-1] == PrimeNu[#+1]&] (* _Jean-François Alcover_, Oct 18 2016 *)

%o (PARI) g(n) = forstep(x=3,n,2,p1=omega(x-1);p2=omega(x+1);if(p1==p2,print(x",")))

%o (Magma) [k: k in [3..265 by 2]| #PrimeDivisors(k-1) eq #PrimeDivisors(k+1)]; // _Marius A. Burtea_, Feb 19 2020

%Y Subsequence of A088070.

%Y Cf. A001221.

%K easy,nonn

%O 1,1

%A _Cino Hilliard_, Mar 03 2006

%E Offset corrected by _Michel Marcus_, Oct 18 2016