21645561 has 8 divisors (see below), whose sum is σ = 32983744. Its totient is φ = 12368880.

The previous prime is 21645551. The next prime is 21645619. The reversal of 21645561 is 16554612.

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

It is a de Polignac number, because none of the positive numbers 2^k-21645561 is a prime.

It is a super-2 number, since 2×21645561² = 937060622009442, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21645511) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 515350 + ... + 515391.

It is an arithmetic number, because the mean of its divisors is an integer number (4122968).

Almost surely, 2^21645561 is an apocalyptic number.

21645561 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21645561 is a deficient number, since it is larger than the sum of its proper divisors (11338183).

21645561 is a wasteful number, since it uses less digits than its factorization.

21645561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1030751.

The product of its digits is 7200, while the sum is 30.

The square root of 21645561 is about 4652.4790166104. The cubic root of 21645561 is about 278.6910030854.

The spelling of 21645561 in words is "twenty-one million, six hundred forty-five thousand, five hundred sixty-one".