OFFSET
1,1
COMMENTS
Let p be an odd prime. If d > p+1 is a divisor of p^2 - p - 2 such that d+1-p is prime, then p*(d+1-p) is in the sequence. - Robert Israel, May 03 2019
LINKS
Robert Israel, Table of n, a(n) for n = 1..500
MAPLE
filter:= proc(n) uses numtheory;
if isprime(n) then return false fi;
type(sigma(n)/(n-phi(n)), integer)
end proc:
select(filter, [seq(seq(4*i+j, j=[0, 1, 3]), i=1..20000)]); # Robert Israel, May 03 2019
MATHEMATICA
Do[ If[ !PrimeQ[ n ], If[ Mod[ n, 4 ]! = 0, If[ Mod[ DivisorSigma[ 1, n ], n-EulerPhi[ n ] ] == 0, Print[ n ] ] ], {n, 1, 5000} ]
PROG
(Sage) [n for n in (1..50000) if not mod(n, 4)==2 and not is_prime(n) and mod(sigma(n), n - euler_phi(n))==0] # G. C. Greubel, May 03 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Robert G. Wilson v, Jun 30 2000
STATUS
approved