21661987920 has 160 divisors (see below), whose sum is σ = 68766312960. Its totient is φ = 5638348800.

The previous prime is 21661987901. The next prime is 21661987931. The reversal of 21661987920 is 2978916612.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 4520886 + ... + 4525674.

It is an arithmetic number, because the mean of its divisors is an integer number (429789456).

Almost surely, 2^21661987920 is an apocalyptic number.

21661987920 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 21661987920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34383156480).

21661987920 is an abundant number, since it is smaller than the sum of its proper divisors (47104325040).

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

21661987920 is a wasteful number, since it uses less digits than its factorization.

21661987920 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5253 (or 5247 counting only the distinct ones).

The product of its (nonzero) digits is 653184, while the sum is 51.

The spelling of 21661987920 in words is "twenty-one billion, six hundred sixty-one million, nine hundred eighty-seven thousand, nine hundred twenty".