OFFSET
1,2
COMMENTS
Erdős & Mirsky call these B-numbers.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
P. Erdős and L. Mirsky, The distribution of values of the divisor function d(n), Proc. London Math. Soc. 2 (1952), pp. 257-271.
FORMULA
Erdős & Mirsky show that there are exp((k + o(1)) sqrt(log x)/log log x) members of this sequence below x, where k = Pi * sqrt(8/3) = 5.130....
PROG
(PARI) is(n)=my(e=valuation(n, 2), e1); if(!isprime(e+1), return(n==1)); n>>=e; forprime(p=3, , if(n==1, return(1)); e1=valuation(n, p); if(!isprime(e1+1) || e1 > e, return(0)); n/=p^e1; e = e1)
CROSSREFS
KEYWORD
nonn
AUTHOR
Charles R Greathouse IV, Dec 07 2012
STATUS
approved