21561745 has 8 divisors (see below), whose sum is σ = 26063784. Its totient is φ = 17122944.

The previous prime is 21561737. The next prime is 21561769. The reversal of 21561745 is 54716512.

It can be written as a sum of positive squares in 4 ways, for example, as 4064256 + 17497489 = 2016^2 + 4183^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21561745 - 2³ = 21561737 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 15054 + ... + 16423.

It is an arithmetic number, because the mean of its divisors is an integer number (3257973).

Almost surely, 2^21561745 is an apocalyptic number.

It is an amenable number.

21561745 is a deficient number, since it is larger than the sum of its proper divisors (4502039).

21561745 is a wasteful number, since it uses less digits than its factorization.

21561745 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 31619.

The product of its digits is 8400, while the sum is 31.

The square root of 21561745 is about 4643.4626088728. The cubic root of 21561745 is about 278.3308217669.

The spelling of 21561745 in words is "twenty-one million, five hundred sixty-one thousand, seven hundred forty-five".