21131604 has 120 divisors (see below), whose sum is σ = 61898760. Its totient is φ = 6241536.

The previous prime is 21131597. The next prime is 21131651. The reversal of 21131604 is 40613112.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 3992004 + 17139600 = 1998^2 + 4140^2 .

It is a super-2 number, since 2×21131604² = 893089375225632, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (18).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 122062 + ... + 122234.

It is an arithmetic number, because the mean of its divisors is an integer number (515823).

Almost surely, 2^21131604 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21131604, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30949380).

21131604 is an abundant number, since it is smaller than the sum of its proper divisors (40767156).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21131604 is a wasteful number, since it uses less digits than its factorization.

21131604 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 231 (or 220 counting only the distinct ones).

The product of its (nonzero) digits is 144, while the sum is 18.

The square root of 21131604 is about 4596.9124420637. The cubic root of 21131604 is about 276.4675440171.

Adding to 21131604 its reverse (40613112), we get a palindrome (61744716).

The spelling of 21131604 in words is "twenty-one million, one hundred thirty-one thousand, six hundred four".