21261240 has 384 divisors, whose sum is σ = 94348800. Its totient is φ = 4008960.

The previous prime is 21261239. The next prime is 21261241. The reversal of 21261240 is 4216212.

21261240 is a `hidden beast` number, since 2 + 12 + 612 + 40 = 666.

It is an interprime number because it is at equal distance from previous prime (21261239) and next prime (21261241).

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

It is a zygodrome in base 3.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21261241) by changing a digit.

It is a polite number, since it can be written in 95 ways as a sum of consecutive naturals, for example, 360331 + ... + 360389.

It is an arithmetic number, because the mean of its divisors is an integer number (245700).

Almost surely, 2^21261240 is an apocalyptic number.

21261240 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 21261240, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (47174400).

21261240 is an abundant number, since it is smaller than the sum of its proper divisors (73087560).

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

21261240 is a wasteful number, since it uses less digits than its factorization.

21261240 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 107 (or 100 counting only the distinct ones).

The product of its (nonzero) digits is 192, while the sum is 18.

The square root of 21261240 is about 4610.9912166475. The cubic root of 21261240 is about 277.0317401191.

Adding to 21261240 its reverse (4216212), we get a palindrome (25477452).

The spelling of 21261240 in words is "twenty-one million, two hundred sixty-one thousand, two hundred forty".