21651220 has 24 divisors (see below), whose sum is σ = 45726408. Its totient is φ = 8611200.

The previous prime is 21651209. The next prime is 21651221. The reversal of 21651220 is 2215612.

It can be written as a sum of positive squares in 4 ways, for example, as 2256004 + 19395216 = 1502^2 + 4404^2 .

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21651221) 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, 630 + ... + 6610.

It is an arithmetic number, because the mean of its divisors is an integer number (1905267).

Almost surely, 2^21651220 is an apocalyptic number.

21651220 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21651220 is an abundant number, since it is smaller than the sum of its proper divisors (24075188).

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

21651220 is a wasteful number, since it uses less digits than its factorization.

21651220 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6171 (or 6169 counting only the distinct ones).

The product of its (nonzero) digits is 240, while the sum is 19.

The square root of 21651220 is about 4653.0871472604. The cubic root of 21651220 is about 278.7152878947.

Adding to 21651220 its reverse (2215612), we get a palindrome (23866832).

The spelling of 21651220 in words is "twenty-one million, six hundred fifty-one thousand, two hundred twenty".