21312540 has 72 divisors (see below), whose sum is σ = 65126880. Its totient is φ = 5641344.

The previous prime is 21312539. The next prime is 21312587. The reversal of 21312540 is 4521312.

It is a super-2 number, since 2×21312540² = 908448722503200, 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 23 ways as a sum of consecutive naturals, for example, 29706 + ... + 30414.

It is an arithmetic number, because the mean of its divisors is an integer number (904540).

Almost surely, 2^21312540 is an apocalyptic number.

21312540 is a gapful number since it is divisible by the number (20) 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 21312540, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32563440).

21312540 is an abundant number, since it is smaller than the sum of its proper divisors (43814340).

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

21312540 is a wasteful number, since it uses less digits than its factorization.

21312540 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 891 (or 886 counting only the distinct ones).

The product of its (nonzero) digits is 240, while the sum is 18.

The square root of 21312540 is about 4616.5506603957. The cubic root of 21312540 is about 277.2543723730.

Adding to 21312540 its reverse (4521312), we get a palindrome (25833852).

The spelling of 21312540 in words is "twenty-one million, three hundred twelve thousand, five hundred forty".