OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..100 (terms 1..51 from Harry J. Smith)
EXAMPLE
441 is a term because sigma(441) - usigma(441) = 241, a prime.
MATHEMATICA
fun[p_, e_] := (p^(e+1)-1)/(p-1); nusigma[1] = 0; nusigma[n_] := Times @@ (fun @@@ (f = FactorInteger[n])) - Times @@ (1 + Power @@@ f); s = {}; pm = 0; Do[If[ PrimeQ[(p = nusigma[n])] && p > pm, pm = p; AppendTo[s, n] ], {n, 1, 10^5}]; s (* Amiram Eldar, Sep 24 2019 *)
PROG
(PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d));
a=0; for(n=1, 50000, x=sigma(n)-u(n); if(isprime(x), b=x; if(b>a, a=b; print(n))))
(PARI) u(n) = sumdiv(n, d, if(gcd(d, n/d)==1, d))
{ n=-1; a=0; for (m=1, 10^9, if(isprime(b=sigma(m) - u(m)), if(b>a, a=b; write("b063760.txt", n++, " ", m); if (n==50, break))) ) } \\ Harry J. Smith, Aug 30 2009
CROSSREFS
KEYWORD
nonn
AUTHOR
Jason Earls, Aug 24 2001
EXTENSIONS
Six more terms from Harry J. Smith, Aug 30 2009
Offset corrected by Amiram Eldar, Sep 24 2019
STATUS
approved