21081720 has 64 divisors (see below), whose sum is σ = 68999040. Its totient is φ = 5110400.

The previous prime is 21081707. The next prime is 21081727. The reversal of 21081720 is 2718012.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21081727) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 6666 + ... + 9305.

It is an arithmetic number, because the mean of its divisors is an integer number (1078110).

Almost surely, 2^21081720 is an apocalyptic number.

21081720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21081720 is an abundant number, since it is smaller than the sum of its proper divisors (47917320).

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

21081720 is a wasteful number, since it uses less digits than its factorization.

21081720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15996 (or 15992 counting only the distinct ones).

The product of its (nonzero) digits is 224, while the sum is 21.

The square root of 21081720 is about 4591.4834204209. The cubic root of 21081720 is about 276.2498263070.

Adding to 21081720 its reverse (2718012), we get a palindrome (23799732).

It can be divided in two parts, 2108 and 1720, that added together give a triangular number (3828 = T₈₇).

The spelling of 21081720 in words is "twenty-one million, eighty-one thousand, seven hundred twenty".