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A210495
Numbers n such that d(n)*n + 1 is prime, d(n) = number of divisors of n.
1
1, 2, 3, 4, 5, 10, 11, 12, 15, 18, 22, 23, 24, 27, 29, 30, 32, 34, 39, 41, 42, 45, 52, 53, 54, 56, 57, 58, 63, 64, 68, 69, 76, 83, 84, 87, 89, 93, 96, 108, 110, 113, 115, 131, 142, 144, 147, 150, 152, 153, 156, 162, 165, 168, 170, 172, 173, 175, 177
OFFSET
1,2
COMMENTS
This is the union of Sophie Germain primes and Sophie Germain nonprimes, so it might be called "Sophie Germain numbers".
LINKS
J. S. Gerasimov, Sophie Germain nonprimes [title corrected], SeqFan mailing list, Jan 15 2013.
MAPLE
A210495 := proc(n)
option remember;
local a;
if n = 1 then
1 ;
else
for a from procname(n-1)+1 do
if isprime(numtheory[tau](a)*a+1) then
return a;
end if;
end do:
end if;
end proc: # R. J. Mathar, Jan 27 2013
MATHEMATICA
Select[Range[200], PrimeQ[# DivisorSigma[0, #]+1]&] (* Harvey P. Dale, Aug 26 2013 *)
PROG
(PARI) is(n)=isprime(numdiv(n)*n+1) \\ Charles R Greathouse IV, Jan 24 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved