A355177 Numbers k such that omega(k^2 - 1) = omega(k) where omega(k) = A001221(k).

2, 3, 6, 10, 12, 15, 18, 24, 26, 28, 33, 48, 63, 66, 70, 72, 78, 80, 82, 84, 90, 105, 108, 110, 114, 120, 130, 132, 140, 156, 165, 170, 174, 182, 190, 192, 195, 222, 234, 238, 242, 252, 258, 264, 273, 276, 280, 290, 294, 306, 308, 310, 315, 318, 336, 342, 345, 350, 354, 357, 366, 372, 374, 378, 380, 385, 396, 399

Offset: 1

Author

Juri-Stepan Gerasimov, Jun 22 2022

Keywords

• nonn
• easy

Examples

			10 is in the sequence because A001221(10^2 - 1) = A001221(3*3*11) = 2 and A001221(10) = A001221(2*5) = 2.
		

Crossrefs

Cf. A001221, A082863, A190789.

Programs

Magma

[n: n in [2..400] | #PrimeDivisors(n^2-1) eq #PrimeDivisors(n)];

Mathematica

Select[Range[400], PrimeNu[#] == PrimeNu[#^2 - 1] &] (* Amiram Eldar, Jun 22 2022 *)

PARI

is(n)=omega(n)==omega(n-1)+omega(n+1)-(n%2) \\ Charles R Greathouse IV, Jul 01 2022

Formula

A001221(a(n)) = A082863(a(n)).