21720725 has 36 divisors (see below), whose sum is σ = 30021516. Its totient is φ = 15575040.

The previous prime is 21720709. The next prime is 21720737. The reversal of 21720725 is 52702712.

It can be written as a sum of positive squares in 18 ways, for example, as 19280881 + 2439844 = 4391^2 + 1562^2 .

It is not a de Polignac number, because 21720725 - 2⁴ = 21720709 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21720694 and 21720703.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 223877 + ... + 223973.

It is an arithmetic number, because the mean of its divisors is an integer number (833931).

Almost surely, 2^21720725 is an apocalyptic number.

21720725 is a gapful number since it is divisible by the number (25) formed by its first and last digit.

It is an amenable number.

21720725 is a deficient number, since it is larger than the sum of its proper divisors (8300791).

21720725 is a wasteful number, since it uses less digits than its factorization.

21720725 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 186 (or 168 counting only the distinct ones).

The product of its (nonzero) digits is 1960, while the sum is 26.

The square root of 21720725 is about 4660.5498602633. The cubic root of 21720725 is about 279.0132143465.

The spelling of 21720725 in words is "twenty-one million, seven hundred twenty thousand, seven hundred twenty-five".