OFFSET
1,1
COMMENTS
A005820 is a subsequence.
101010 is the least squarefree term. For 6 <= k <= 8, the smallest squarefree terms with k prime factors are 101010 = 2*3*5*7*13*37, 35574630 = 2*3*5*7*13*83*157, and 401738610 = 2*3*5*7*13*31*47*101.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..1210 (terms below 4*10^9)
EXAMPLE
120 is a term because sigma(sigma(120) - 240) = sigma(360 - 240) = sigma(120).
MATHEMATICA
Select[Range[1.3*^6], (d = DivisorSigma[1, #]; d > 2 # && DivisorSigma[1, d - 2 #] == d) &] (* Giovanni Resta, Sep 18 2017 *)
PROG
(PARI) s(n) = sigma(n);
isok(n) = s(n)>2*n && s(s(n)-2*n)==s(n);
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Sep 15 2017
STATUS
approved