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Numbers k such that the number of divisors of k^2-1 equals the number of divisors of k.
1

%I #11 Jul 17 2019 06:50:18

%S 2,6,40,54,66,70,78,80,88,110,114,128,130,182,216,222,238,250,258,276,

%T 310,318,354,366,384,402,410,418,438,456,488,498,552,594,598,616,624,

%U 632,654,678,680,682,696,726,760,762,774,786,808,830,870,906,924,928

%N Numbers k such that the number of divisors of k^2-1 equals the number of divisors of k.

%H Amiram Eldar, <a href="/A068207/b068207.txt">Table of n, a(n) for n = 1..10000</a>

%F k such that A000005(k^2-1) = A000005(k).

%t Select[Range[1000], DivisorSigma[0, #] == DivisorSigma[0, #^2-1] &] (* _Amiram Eldar_, Jul 17 2019 *)

%Y Cf. A000005.

%K easy,nonn

%O 1,1

%A _Benoit Cloitre_, Mar 23 2002