A115165 Odd numbers k such that k-1 and k+1 have the same number of distinct prime divisors.

3, 11, 13, 19, 21, 23, 25, 27, 35, 37, 39, 45, 47, 49, 51, 53, 55, 57, 73, 75, 81, 87, 93, 95, 97, 99, 105, 107, 117, 123, 131, 135, 139, 143, 145, 147, 155, 159, 161, 163, 165, 169, 177, 181, 193, 195, 201, 207, 213, 215, 217, 221, 225, 229, 239, 243, 249, 259, 265

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Mathematica

Select[Range[1, 301, 2], PrimeNu[#-1] == PrimeNu[#+1]&] (* Jean-François Alcover, Oct 18 2016 *)

Prog

(PARI) g(n) = forstep(x=3, n, 2, p1=omega(x-1); p2=omega(x+1); if(p1==p2, print(x", ")))
(Magma) [k: k in [3..265 by 2]| #PrimeDivisors(k-1) eq #PrimeDivisors(k+1)]; // Marius A. Burtea, Feb 19 2020

Crossrefs

Subsequence of A088070.

Cf. A001221.

Sequence in context: A160025 A045427 A373726 * A050583 A045428 A338445

Adjacent sequences: A115162 A115163 A115164 * A115166 A115167 A115168

Keyword

easy,nonn

Author

Cino Hilliard, Mar 03 2006

Extensions

Offset corrected by Michel Marcus, Oct 18 2016

Status

approved