21501818 has 16 divisors (see below), whose sum is σ = 35932680. Its totient is φ = 9581376.

The previous prime is 21501811. The next prime is 21501827. The reversal of 21501818 is 81810512.

It can be written as a sum of positive squares in 4 ways, for example, as 18722929 + 2778889 = 4327^2 + 1667^2 .

It is a Harshad number since it is a multiple of its sum of digits (26).

It is not an unprimeable number, because it can be changed into a prime (21501811) 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, 13505 + ... + 15012.

Almost surely, 2^21501818 is an apocalyptic number.

21501818 is a deficient number, since it is larger than the sum of its proper divisors (14430862).

21501818 is a wasteful number, since it uses less digits than its factorization.

21501818 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 28561.

The product of its (nonzero) digits is 640, while the sum is 26.

The square root of 21501818 is about 4637.0052835855. The cubic root of 21501818 is about 278.0727256550.

The spelling of 21501818 in words is "twenty-one million, five hundred one thousand, eight hundred eighteen".