21004561 has 6 divisors (see below), whose sum is σ = 21267650. Its totient is φ = 20744688.

The previous prime is 21004549. The next prime is 21004579. The reversal of 21004561 is 16540012.

It can be written as a sum of positive squares in only one way, i.e., 13950225 + 7054336 = 3735^2 + 2656^2 .

It is not a de Polignac number, because 21004561 - 2¹³ = 20996369 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21004541) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 5365 + ... + 8413.

Almost surely, 2^21004561 is an apocalyptic number.

It is an amenable number.

21004561 is a deficient number, since it is larger than the sum of its proper divisors (263089).

21004561 is an frugal number, since it uses more digits than its factorization.

21004561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3215 (or 3132 counting only the distinct ones).

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

The square root of 21004561 is about 4583.0733138365. The cubic root of 21004561 is about 275.9123899274.

Adding to 21004561 its reverse (16540012), we get a palindrome (37544573).

The spelling of 21004561 in words is "twenty-one million, four thousand, five hundred sixty-one".