21520321 has 3 divisors (see below), whose sum is σ = 21524961.
Its totient is φ = 21515682.
The previous prime is 21520307. The next prime is 21520361. The reversal of 21520321 is 12302512.
Multipling 21520321 by its sum of digits (16), we get a square (344325136 = 185562).
Adding to 21520321 its reverse (12302512), we get a palindrome (33822833).
The square root of 21520321 is 4639.
It is a perfect power (a square), and thus also a powerful number.
21520321 is nontrivially palindromic in base 15.
It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.
It is not a de Polignac number, because 21520321 - 27 = 21520193 is a prime.
It is a Duffinian number.
It is a junction number, because it is equal to n+sod(n) for n = 21520295 and 21520304.
It is not an unprimeable number, because it can be changed into a prime (21520361) by changing a digit.
It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 2320 + ... + 6958.
It is an arithmetic number, because the mean of its divisors is an integer number (7174987).
Almost surely, 221520321 is an apocalyptic number.
21520321 is the 4639-th square number.
21520321 is the 2320-th centered octagonal number.
It is an amenable number.
21520321 is a deficient number, since it is larger than the sum of its proper divisors (4640).
21520321 is an frugal number, since it uses more digits than its factorization.
21520321 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 9278 (or 4639 counting only the distinct ones).
The product of its (nonzero) digits is 120, while the sum is 16.
The cubic root of 21520321 is about 278.1524662687.
The spelling of 21520321 in words is "twenty-one million, five hundred twenty thousand, three hundred twenty-one".