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A067893 Nonprimes k such that tau(k)*sigma(k) < prime(k) where tau(k) = A000005(k) and sigma(k) = A000203(k). 1

%I #21 Jul 18 2019 10:16:46

%S 1,25,49,77,85,91,95,115,119,121,123,125,129,133,141,143,145,155,159,

%T 161,169,177,183,185,187,194,201,202,203,205,206,209,213,214,215,217,

%U 218,219,221,226,235,237,247,249,253,254,259,262,265,267,274,278,287

%N Nonprimes k such that tau(k)*sigma(k) < prime(k) where tau(k) = A000005(k) and sigma(k) = A000203(k).

%H Amiram Eldar, <a href="/A067893/b067893.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[300],!PrimeQ[#]&&DivisorSigma[0,#]DivisorSigma[1,#]<Prime[ #]&](* _Harvey P. Dale_, Nov 09 2017 *)

%o (PARI) isok(k) = !isprime(k) && (numdiv(k)*sigma(k) < prime(k)); \\ _Michel Marcus_, Jul 18 2019

%Y Cf. A000005, A000040, A000203.

%K easy,nonn

%O 1,2

%A _Benoit Cloitre_, Mar 02 2002

%E Typo in definition corrected by _Jonathan Sondow_, Nov 20 2012

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Last modified March 28 13:25 EDT 2024. Contains 371254 sequences. (Running on oeis4.)