21521810 has 16 divisors (see below), whose sum is σ = 38814552. Its totient is φ = 8592000.

The previous prime is 21521803. The next prime is 21521821. The reversal of 21521810 is 1812512.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 21464689 + 57121 = 4633^2 + 239^2 .

It is a super-2 number, since 2×21521810² = 926376611352200, which contains 22 as substring.

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, 4220 + ... + 7800.

Almost surely, 2^21521810 is an apocalyptic number.

21521810 is a deficient number, since it is larger than the sum of its proper divisors (17292742).

21521810 is a wasteful number, since it uses less digits than its factorization.

21521810 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4189.

The product of its (nonzero) digits is 160, while the sum is 20.

The square root of 21521810 is about 4639.1604843980. The cubic root of 21521810 is about 278.1588812825.

The spelling of 21521810 in words is "twenty-one million, five hundred twenty-one thousand, eight hundred ten".