21291808 has 12 divisors (see below), whose sum is σ = 41918310. Its totient is φ = 10645888.

The previous prime is 21291757. The next prime is 21291821. The reversal of 21291808 is 80819212.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 21049744 + 242064 = 4588^2 + 492^2 .

It is a self number, because there is not a number n which added to its sum of digits gives 21291808.

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 332653 + ... + 332716.

Almost surely, 2^21291808 is an apocalyptic number.

It is an amenable number.

21291808 is a deficient number, since it is larger than the sum of its proper divisors (20626502).

21291808 is an equidigital number, since it uses as much as digits as its factorization.

21291808 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 665379 (or 665371 counting only the distinct ones).

The product of its (nonzero) digits is 2304, while the sum is 31.

The square root of 21291808 is about 4614.3047146889. The cubic root of 21291808 is about 277.1644424933.

The spelling of 21291808 in words is "twenty-one million, two hundred ninety-one thousand, eight hundred eight".