OFFSET
1,1
COMMENTS
From Robert Israel, Aug 12 2016: (Start)
d(m) is prime iff m = p^k where p is prime and k+1 is prime.
For such m, sigma(m) = 1 + p + ... + p^k = (p*m-1)/(p-1).
The sequence contains 2^(q-1) for q in A054723,
3^(q-1) for q prime but not in A028491,
5^(q-1) for q prime but not in A004061,
7^(q-1) for q prime but not in A004063, etc.
In particular, it contains all odd primes. (End)
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
49 is a term because A000005(49) = 3 is prime while sigma(49) = 57 is not.
MAPLE
N:= 1000: # to get all terms <= N
P:= select(isprime, {2, seq(p, p=3..N, 2)}):
fp:= proc(p) local q, res;
q:= 2;
res:= NULL;
while p^(q-1) <= N do
if not isprime((p^q-1)/(p-1)) then res:= res, p^(q-1) fi;
q:= nextprime(q);
od;
res;
end proc:
sort(convert(map(fp, P), list)); # Robert Israel, Aug 12 2016
PROG
(PARI) lista(nn) = for(n=1, nn, if(isprime(numdiv(n)) && !isprime(sigma(n)), print1(n, ", ")));
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Altug Alkan, Aug 12 2016
STATUS
approved