21491820 has 144 divisors (see below), whose sum is σ = 76684608. Its totient is φ = 4769280.

The previous prime is 21491819. The next prime is 21491837. The reversal of 21491820 is 2819412.

It is a congruent number.

It is an unprimeable number.

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 47 ways as a sum of consecutive naturals, for example, 46390 + ... + 46850.

It is an arithmetic number, because the mean of its divisors is an integer number (532532).

Almost surely, 2^21491820 is an apocalyptic number.

21491820 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21491820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (38342304).

21491820 is an abundant number, since it is smaller than the sum of its proper divisors (55192788).

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

21491820 is a wasteful number, since it uses less digits than its factorization.

21491820 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 520 (or 515 counting only the distinct ones).

The product of its (nonzero) digits is 1152, while the sum is 27.

The square root of 21491820 is about 4635.9270917477. The cubic root of 21491820 is about 278.0296191894.

It can be divided in two parts, 2149 and 1820, that added together give a square (3969 = 63²).

The spelling of 21491820 in words is "twenty-one million, four hundred ninety-one thousand, eight hundred twenty".