A066025 - OEIS

%I #13 Jun 15 2018 22:08:52

%S 5,12,14,70,88,125,180,204,206,220,957,1253,1334,1364,1634,1673,1750,

%T 1888,1919,2685,2759,2958,2974,3127,4364,5191,7615,8142,8632,9114,

%U 11219,12000,12035,14664,14841,18414,18762,18873,19358,20118,20145,20712,24957,25194

%N Numbers k such that gcd(sigma(k), sigma(k+1)) > k.

%H Harry J. Smith, <a href="/A066025/b066025.txt">Table of n, a(n) for n = 1..1000</a>

%e gcd(sigma(12), sigma(13)) = gcd(28, 14) = 14 > 12, so 12 is in the sequence.

%t Select[Range[1, 10^5], GCD[DivisorSigma[1, # ], DivisorSigma[ 1, # + 1]] > # &]

%o (PARI) { n=0; s=1; for (m=1, 10^9, t=s; s=sigma(m + 1); if (gcd(t, s) > m, write("b066025.txt", n++, " ", m); if (n==1000, return)) ) } \\ _Harry J. Smith_, Nov 07 2009

%Y Cf. A000010, A000203.

%K nonn

%O 1,1

%A _Joseph L. Pe_, Dec 11 2001

%E Definition and Example edited by _Harry J. Smith_, Nov 07 2009