20721990 has 32 divisors (see below), whose sum is σ = 50196960. Its totient is φ = 5474304.

The previous prime is 20721983. The next prime is 20721991. The reversal of 20721990 is 9912702.

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20721991) by changing a digit.

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 15 ways as a sum of consecutive naturals, for example, 102 + ... + 6438.

It is an arithmetic number, because the mean of its divisors is an integer number (1568655).

Almost surely, 2^20721990 is an apocalyptic number.

20721990 is an abundant number, since it is smaller than the sum of its proper divisors (29474970).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

20721990 is a wasteful number, since it uses less digits than its factorization.

20721990 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6456.

The product of its (nonzero) digits is 2268, while the sum is 30.

The square root of 20721990 is about 4552.1412543989. The cubic root of 20721990 is about 274.6695314083.

The spelling of 20721990 in words is "twenty million, seven hundred twenty-one thousand, nine hundred ninety".