OFFSET
2,1
COMMENTS
LINKS
Amiram Eldar, Table of n, a(n) for n = 2..3000
K. Alladi and P. Erdős, On an additive arithmetic function, Pacific J. Math., Volume 71, Number 2 (1977), 275-294.
FORMULA
a(p) = p, for p prime.
EXAMPLE
a(10) = 30 since A001414(30)=10 and 30 is divisible by 10, and 30/10=3 is squarefree and prime to 10.
PROG
(PARI) sopfr(n) = {my(f=factor(n)); sum(k=1, #f~, f[k, 1]*f[k, 2]); }
first(n) = {my(k=1); while (sopfr(k) != n, k++); k; }
last(n) = polcoeff((1+x+2*x^2+x^4)/(1-3*x^3) + O(x^(n + 3)), n);
a(n) = {na = first(n); nb = last(n); for (m=na, nb, if ((sopfr(m) == n) && (! (m % n)) && issquarefree(m/n) && (gcd(m/n, n) == 1), return(m)); ); }
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Jan 08 2016
STATUS
approved