21661920 has 216 divisors, whose sum is σ = 86270184. Its totient is φ = 4935168.

The previous prime is 21661903. The next prime is 21661931. The reversal of 21661920 is 2916612.

21661920 is a `hidden beast` number, since 21 + 6 + 619 + 20 = 666.

It is a hoax number, since the sum of its digits (27) coincides with the sum of the digits of its distinct prime factors.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 70407 + ... + 70713.

It is an arithmetic number, because the mean of its divisors is an integer number (399399).

Almost surely, 2^21661920 is an apocalyptic number.

21661920 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 21661920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (43135092).

21661920 is an abundant number, since it is smaller than the sum of its proper divisors (64608264).

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

21661920 is a wasteful number, since it uses less digits than its factorization.

21661920 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 342 (or 324 counting only the distinct ones).

The product of its (nonzero) digits is 1296, while the sum is 27.

The square root of 21661920 is about 4654.2367795375. The cubic root of 21661920 is about 278.7611938905.

The spelling of 21661920 in words is "twenty-one million, six hundred sixty-one thousand, nine hundred twenty".