21240305 has 4 divisors (see below), whose sum is σ = 25488372. Its totient is φ = 16992240.

The previous prime is 21240283. The next prime is 21240311. The reversal of 21240305 is 50304212.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 2819041 + 18421264 = 1679^2 + 4292^2 .

It is not a de Polignac number, because 21240305 - 2⁸ = 21240049 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2124026 + ... + 2124035.

It is an arithmetic number, because the mean of its divisors is an integer number (6372093).

Almost surely, 2^21240305 is an apocalyptic number.

It is an amenable number.

21240305 is a deficient number, since it is larger than the sum of its proper divisors (4248067).

21240305 is an equidigital number, since it uses as much as digits as its factorization.

21240305 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4248066.

The product of its (nonzero) digits is 240, while the sum is 17.

The square root of 21240305 is about 4608.7205382839. The cubic root of 21240305 is about 276.9407833032.

Adding to 21240305 its reverse (50304212), we get a palindrome (71544517).

It can be divided in two parts, 21240 and 305, that multiplied together give a triangular number (6478200 = T₃₅₉₉).

The spelling of 21240305 in words is "twenty-one million, two hundred forty thousand, three hundred five".