21623721 has 16 divisors (see below), whose sum is σ = 33055104. Its totient is φ = 12317184.

The previous prime is 21623699. The next prime is 21623731. The reversal of 21623721 is 12732612.

It is not a de Polignac number, because 21623721 - 2⁷ = 21623593 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21623691 and 21623700.

It is not an unprimeable number, because it can be changed into a prime (21623731) 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, 5955 + ... + 8871.

It is an arithmetic number, because the mean of its divisors is an integer number (2065944).

Almost surely, 2^21623721 is an apocalyptic number.

21623721 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21623721 is a deficient number, since it is larger than the sum of its proper divisors (11431383).

21623721 is a wasteful number, since it uses less digits than its factorization.

21623721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3280.

The product of its digits is 1008, while the sum is 24.

The square root of 21623721 is about 4650.1312884692. The cubic root of 21623721 is about 278.5972400616.

The spelling of 21623721 in words is "twenty-one million, six hundred twenty-three thousand, seven hundred twenty-one".