21520890 has 60 divisors (see below), whose sum is σ = 58224474. Its totient is φ = 5703696.

The previous prime is 21520879. The next prime is 21520897. The reversal of 21520890 is 9802512.

It can be written as a sum of positive squares in only one way, i.e., 19368801 + 2152089 = 4401^2 + 1467^2 .

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a Curzon number.

It is a nialpdrome in base 9.

It is a zygodrome in base 9.

It is not an unprimeable number, because it can be changed into a prime (21520897) by changing a digit.

It is a polite number, since it can be written in 29 ways as a sum of consecutive naturals, for example, 131949 + ... + 132111.

Almost surely, 2^21520890 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21520890, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29112237).

21520890 is an abundant number, since it is smaller than the sum of its proper divisors (36703584).

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

21520890 is an equidigital number, since it uses as much as digits as its factorization.

21520890 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 345 (or 173 counting only the distinct ones).

The product of its (nonzero) digits is 1440, while the sum is 27.

The square root of 21520890 is about 4639.0613274670. The cubic root of 21520890 is about 278.1549177092.

Multiplying 21520890 by its product of nonzero digits (1440), we get a square (30990081600 = 176040²).

The spelling of 21520890 in words is "twenty-one million, five hundred twenty thousand, eight hundred ninety".