21291720 has 32 divisors (see below), whose sum is σ = 63875520. Its totient is φ = 5677760.

The previous prime is 21291719. The next prime is 21291727. The reversal of 21291720 is 2719212.

It is a Harshad number since it is a multiple of its sum of digits (24).

It is not an unprimeable number, because it can be changed into a prime (21291727) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 88596 + ... + 88835.

It is an arithmetic number, because the mean of its divisors is an integer number (1996110).

Almost surely, 2^21291720 is an apocalyptic number.

21291720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21291720 is an abundant number, since it is smaller than the sum of its proper divisors (42583800).

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

21291720 is a wasteful number, since it uses less digits than its factorization.

21291720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 177445 (or 177441 counting only the distinct ones).

The product of its (nonzero) digits is 504, while the sum is 24.

The square root of 21291720 is about 4614.2951791146. The cubic root of 21291720 is about 277.1640606484.

The spelling of 21291720 in words is "twenty-one million, two hundred ninety-one thousand, seven hundred twenty".