OFFSET
1,1
COMMENTS
We say d is a strong divisor of n iff d is a divisor of n and d > 1.
All primes and squares of primes are in this sequence.
LINKS
EXAMPLE
28 is in the sequence because 28 has 6 divisors {1, 2, 4, 7, 14, 28} therefore 5 strong divisors {2, 4, 7, 14, 28}, 2 + 4 + 7 + 14 + 28 = 55 and 5 divides 55.
MAPLE
filter:= proc(n) local d, t;
d:= numtheory:-divisors(n) minus {1};
convert(d, `+`) mod nops(d) = 0
end proc:
select(filter, [$2..1000]); # Robert Israel, Mar 27 2017
MATHEMATICA
Select[Range[2, 200], Mod[DivisorSigma[1, #1] - 1, DivisorSigma[0, #1] - 1] == 0 &]
PROG
(PARI) for(n=2, 200, if((sigma(n) - 1)%(numdiv(n) - 1)==0, print1(n, ", "))) \\ Indranil Ghosh, Mar 24 2017
(Python)
from sympy.ntheory import divisor_sigma, divisor_count
print([n for n in range(2, 201) if (divisor_sigma(n) - 1)%(divisor_count(n) - 1) == 0]) # Indranil Ghosh, Mar 24 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Mar 24 2017
STATUS
approved