21505851 has 16 divisors (see below), whose sum is σ = 33246720. Its totient is φ = 13713480.

The previous prime is 21505831. The next prime is 21505859. The reversal of 21505851 is 15850512.

It is not a de Polignac number, because 21505851 - 2⁶ = 21505787 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21505859) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 16695 + ... + 17936.

It is an arithmetic number, because the mean of its divisors is an integer number (2077920).

Almost surely, 2^21505851 is an apocalyptic number.

21505851 is a deficient number, since it is larger than the sum of its proper divisors (11740869).

21505851 is a wasteful number, since it uses less digits than its factorization.

21505851 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 34663 (or 34657 counting only the distinct ones).

The product of its (nonzero) digits is 2000, while the sum is 27.

The square root of 21505851 is about 4637.4401343845. The cubic root of 21505851 is about 278.0901101880.

It can be divided in two parts, 2150 and 5851, that added together give a triangular number (8001 = T₁₂₆).

The spelling of 21505851 in words is "twenty-one million, five hundred five thousand, eight hundred fifty-one".