A066692 - OEIS

%I #21 Dec 02 2023 09:12:45

%S 3,5,11,17,29,33,41,55,59,71,85,91,93,101,107,123,137,141,143,149,159,

%T 179,183,185,191,197,201,203,213,215,217,219,227,235,239,243,247,265,

%U 269,281,299,301,303,311,319,321,327,339,341,347,391,393,411,413,415

%N Odd numbers k such that tau(k) = tau(k+2), where tau(k) = A000005(k) is the number of divisors of k.

%H Amiram Eldar, <a href="/A066692/b066692.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harry J. Smith)

%F a(n) = 2*A190276(n) - 1. - _Amiram Eldar_, Dec 02 2023

%t d[n_] := d[n] = DivisorSigma[0, n]; Select[Range[1, 420, 2], d[#] == d[# + 2] &] (* _Amiram Eldar_, Dec 02 2023 *)

%o (PARI) for (n=2,1000, if (numdiv(2*n-1)==numdiv(2*n+1),print1(2*n-1, ", ")))

%o (PARI) { n=0; for (m=2, 10^10, if (numdiv(2*m-1) == numdiv(2*m+1), write("b066692.txt", n++, " ", 2*m-1); if (n==1000, return)) ) } \\ _Harry J. Smith_, Mar 18 2010

%Y Cf. A000005, A190276.

%K nonn,easy

%O 1,1

%A _Jon Perry_, Jan 11 2002

%E Edited by _Dean Hickerson_, Jan 13 2002

%E Offset changed from 0 to 1 by _Harry J. Smith_, Mar 18 2010