21986721 has 15 divisors (see below), whose sum is σ = 32907523. Its totient is φ = 14629680.

The previous prime is 21986711. The next prime is 21986737. The reversal of 21986721 is 12768912.

The square root of 21986721 is 4689.

It is a perfect power (a square), and thus also a powerful number.

It can be written as a sum of positive squares in only one way, i.e., 6305121 + 15681600 = 2511^2 + 3960^2 .

It is not a de Polignac number, because 21986721 - 2¹⁷ = 21855649 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21986711) by changing a digit.

It is a polite number, since it can be written in 14 ways as a sum of consecutive naturals, for example, 41941 + ... + 42461.

Almost surely, 2^21986721 is an apocalyptic number.

21986721 is the 4689-th square number.

21986721 is the 2345-th centered octagonal number.

It is an amenable number.

21986721 is a deficient number, since it is larger than the sum of its proper divisors (10920802).

21986721 is an frugal number, since it uses more digits than its factorization.

21986721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1054 (or 524 counting only the distinct ones).

The product of its digits is 12096, while the sum is 36.

The cubic root of 21986721 is about 280.1475455368.

Multiplying 21986721 by its sum of digits (36), we get a square (791521956 = 28134²).

The spelling of 21986721 in words is "twenty-one million, nine hundred eighty-six thousand, seven hundred twenty-one".