21991990 has 16 divisors (see below), whose sum is σ = 40551840. Its totient is φ = 8582080.

The previous prime is 21991969. The next prime is 21991993. The reversal of 21991990 is 9919912.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21991993) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 26410 + ... + 27229.

It is an arithmetic number, because the mean of its divisors is an integer number (2534490).

Almost surely, 2^21991990 is an apocalyptic number.

21991990 is a deficient number, since it is larger than the sum of its proper divisors (18559850).

21991990 is a wasteful number, since it uses less digits than its factorization.

21991990 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 53687.

The product of its (nonzero) digits is 13122, while the sum is 40.

The square root of 21991990 is about 4689.5618132188. The cubic root of 21991990 is about 280.1699223693.

The spelling of 21991990 in words is "twenty-one million, nine hundred ninety-one thousand, nine hundred ninety".