21505861 has 8 divisors (see below), whose sum is σ = 23506784. Its totient is φ = 19554480.

The previous prime is 21505859. The next prime is 21505889. The reversal of 21505861 is 16850512.

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 21505861 - 2¹ = 21505859 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21505831) 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, 11475 + ... + 13216.

It is an arithmetic number, because the mean of its divisors is an integer number (2938348).

Almost surely, 2^21505861 is an apocalyptic number.

It is an amenable number.

21505861 is a deficient number, since it is larger than the sum of its proper divisors (2000923).

21505861 is a wasteful number, since it uses less digits than its factorization.

21505861 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 24771.

The product of its (nonzero) digits is 2400, while the sum is 28.

The square root of 21505861 is about 4637.4412125654. The cubic root of 21505861 is about 278.0901532910.

It can be divided in two parts, 21505 and 861, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21505861 in words is "twenty-one million, five hundred five thousand, eight hundred sixty-one".