21405861 has 24 divisors (see below), whose sum is σ = 31711680. Its totient is φ = 13910400.

The previous prime is 21405829. The next prime is 21405871. The reversal of 21405861 is 16850412.

It is not a de Polignac number, because 21405861 - 2⁵ = 21405829 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21405871) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 88701 + ... + 88941.

It is an arithmetic number, because the mean of its divisors is an integer number (1321320).

Almost surely, 2^21405861 is an apocalyptic number.

It is an amenable number.

21405861 is a deficient number, since it is larger than the sum of its proper divisors (10305819).

21405861 is a wasteful number, since it uses less digits than its factorization.

21405861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 457 (or 454 counting only the distinct ones).

The product of its (nonzero) digits is 1920, while the sum is 27.

The square root of 21405861 is about 4626.6468419364. The cubic root of 21405861 is about 277.6584534547.

It can be divided in two parts, 2140 and 5861, that added together give a triangular number (8001 = T₁₂₆).

The spelling of 21405861 in words is "twenty-one million, four hundred five thousand, eight hundred sixty-one".