OFFSET
1,1
COMMENTS
FORMULA
a(n) = A019279(n) + 1. - Michel Marcus, Feb 09 2020
EXAMPLE
17 is in the sequence because sigma(sigma(17-1)) = 32 = 2*(17-1).
MAPLE
with(numtheory): A256438:=n->`if`(sigma(sigma(n-1)) = 2*(n-1), n, NULL): seq(A256438(n), n=2..10^5); # Wesley Ivan Hurt, Mar 30 2015
MATHEMATICA
Select[Range@ 1000000, DivisorSigma[1, DivisorSigma[1, # - 1]] == 2 (# - 1) &] (* Michael De Vlieger, Mar 29 2015 *)
PROG
(Magma) [n: n in[2..10000000] | SumOfDivisors(SumOfDivisors(n-1)) eq 2*(n-1)]
(PARI) isok(m) = sigma(sigma(m-1)) == 2*(m-1); \\ Michel Marcus, Feb 09 2020
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jaroslav Krizek, Mar 29 2015
STATUS
approved