21720739 has 16 divisors (see below), whose sum is σ = 23857920. Its totient is φ = 19716480.

The previous prime is 21720737. The next prime is 21720749. The reversal of 21720739 is 93702712.

It is a cyclic number.

It is not a de Polignac number, because 21720739 - 2¹ = 21720737 is a prime.

It is a super-2 number, since 2×21720739² = 943581005412242, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (31).

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21720739.

It is not an unprimeable number, because it can be changed into a prime (21720737) 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, 32937 + ... + 33589.

It is an arithmetic number, because the mean of its divisors is an integer number (1491120).

Almost surely, 2^21720739 is an apocalyptic number.

21720739 is a gapful number since it is divisible by the number (29) formed by its first and last digit.

21720739 is a deficient number, since it is larger than the sum of its proper divisors (2137181).

21720739 is a wasteful number, since it uses less digits than its factorization.

21720739 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 750.

The product of its (nonzero) digits is 5292, while the sum is 31.

The square root of 21720739 is about 4660.5513622317. The cubic root of 21720739 is about 279.0132742921.

The spelling of 21720739 in words is "twenty-one million, seven hundred twenty thousand, seven hundred thirty-nine".