OFFSET
1,2
COMMENTS
The terms involve powers of small primes. - Jud McCranie, Nov 29 2001
From Kevin P. Thompson, Jun 20 2022: (Start)
Theorem: Terms that are greater than one must be twice a square.
Proof: Since sigma(k) is odd if and only if k is a square or twice a square, and tau(k) is odd if and only if k is a square, then an odd sum only occurs when k is twice a square, in which case sigma(k) is odd and tau(k) is even. So, these are the only candidates for sigma(k) + tau(k) being prime.
Theorem: No terms are congruent to 4 or 6 (mod 10).
Proof: Since no square ends in 2, 3, 7, or 8, and each term > 1 is twice a square, no term ends in 4 or 6. (End)
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..5000 (first 34 terms from Harry J. Smith, terms 35..276 from Kevin P. Thompson)
EXAMPLE
128 is a term since sigma(128) + tau(128) = 255 + 8 = 263, which is prime.
MATHEMATICA
Do[ If[ PrimeQ[ DivisorSigma[1, n] + DivisorSigma[0, n]], Print[n]], {n, 1, 10^7}]
PROG
(PARI) { n=0; for (m=1, 10^9, if (isprime(sigma(m) + numdiv(m)), write("b064205.txt", n++, " ", m); if (n==100, break)) ) } \\ Harry J. Smith, Sep 10 2009
(Python)
from itertools import count, islice
from sympy import isprime, divisor_sigma as s, divisor_count as t
def agen(): # generator of terms
yield 1
yield from (k for k in (2*i*i for i in count(1)) if isprime(s(k)+t(k)))
print(list(islice(agen(), 30))) # Michael S. Branicky, Jun 20 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Jason Earls, Sep 21 2001
EXTENSIONS
More terms from Robert G. Wilson v, Nov 12 2001
More terms from Labos Elemer, Nov 22 2001
More terms from Jud McCranie, Nov 29 2001
a(28) from Harry J. Smith, Sep 10 2009
STATUS
approved