OFFSET
1,2
COMMENTS
Number of n-digit binary numbers with exactly n divisors.
If p is an odd prime, then the only p-digit binary number having exactly p divisors is 2^(p-1), so a(p) = 1.
Only squares have an odd number of divisors, so for odd values of n, a(n) is the number of numbers in the interval [ceiling(sqrt(2^(n-1))), floor(sqrt(2^n-1))] whose squares have exactly n divisors. The next few odd-indexed terms are a(41) = 1, a(43) = 1, a(45) = 139407, a(47) = 1, and a(49) = 8. - Jon E. Schoenfield, May 26 2018
EXAMPLE
a(1) = 1 because the only number in the interval [2^(1-1), 2^1 - 1] = [1, 1] having exactly 1 divisor is 1.
a(2) = 2 because each of the two numbers in the interval [2^(2-1), 2^2 - 1] = [2, 3] has exactly 2 divisors.
a(8) = 25 because the numbers in the interval [2^(8-1), 2^8 - 1] = [128, 255] having exactly 8 divisors are the 1 number of the form p^7 {i.e., 2^7 = 128}, the 8 numbers of the form p^3 * q {135, 136, 152, 184, 189, 232, 248, 250}, and the 16 numbers of the form p*q*r {130, 138, 154, 165, 170, 174, 182, 186, 190, 195, 222, 230, 231, 238, 246, 255}; 1 + 8 + 16 = 25.
PROG
(PARI) a(n) = sum(k=2^(n-1), 2^n-1, numdiv(k)==n); \\ Michel Marcus, May 26 2018
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Jon E. Schoenfield, May 25 2018
EXTENSIONS
a(26)-a(38) from Giovanni Resta, May 26 2018
a(39) from Jon E. Schoenfield, May 26 2018
a(40)-a(41) from Giovanni Resta, May 27 2018
a(42)-a(47) from Jon E. Schoenfield, May 27 2018
STATUS
approved