21081281904 has 120 divisors (see below), whose sum is σ = 60212552400. Its totient is φ = 6883937280.

The previous prime is 21081281899. The next prime is 21081281909. The reversal of 21081281904 is 40918218012.

It is a happy number.

21081281904 is a `hidden beast` number, since 210 + 81 + 281 + 90 + 4 = 666.

It is an interprime number because it is at equal distance from previous prime (21081281899) and next prime (21081281909).

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21081281909) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 1389399 + ... + 1404489.

It is an arithmetic number, because the mean of its divisors is an integer number (501771270).

Almost surely, 2^21081281904 is an apocalyptic number.

21081281904 is a gapful number since it is divisible by the number (24) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21081281904, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30106276200).

21081281904 is an abundant number, since it is smaller than the sum of its proper divisors (39131270496).

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

21081281904 is a wasteful number, since it uses less digits than its factorization.

21081281904 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15303 (or 15294 counting only the distinct ones).

The product of its (nonzero) digits is 9216, while the sum is 36.

Adding to 21081281904 its reverse (40918218012), we get a palindrome (61999499916).

The spelling of 21081281904 in words is "twenty-one billion, eighty-one million, two hundred eighty-one thousand, nine hundred four".