OFFSET
1,1
COMMENTS
Numbers that occur twice in the sequence include 7560, 816000, 2709504, 31752000. Are there infinitely many? Does any number occur more than twice? - Robert Israel, Sep 12 2018
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
EXAMPLE
For n = 9 the sum of the divisors of 9 is 1 + 3 + 9 = 13, and the 9th prime is 23, and the sum of the divisors of 23 is 1 + 23 = 24, and 13*24 = 312, so a(9) = 312.
On the other hand 9*23 = 207, and the sum of the divisors of 207 is 1 + 3 + 9 + 23 + 69 + 207 = 312, so a(9) = 312.
MAPLE
f:= n -> numtheory:-sigma(n)*(1+ithprime(n)):
map(f, [$1..100]); # Robert Israel, Sep 12 2018
MATHEMATICA
Table[DivisorSigma[1, n]*DivisorSigma[1, Prime[n]], {n, 1, 50}] (* G. C. Greubel, Apr 27 2016 *)
PROG
(PARI) a(n) = sigma(n)*sigma(prime(n)); \\ Michel Marcus, Apr 27 2016
(Magma) [SumOfDivisors(n)*SumOfDivisors(NthPrime(n)): n in [1..50]]; // Vincenzo Librandi, Sep 13 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Apr 26 2016
STATUS
approved