21398561 has 2 divisors, whose sum is σ = 21398562. Its totient is φ = 21398560.

The previous prime is 21398557. The next prime is 21398569. The reversal of 21398561 is 16589312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19088161 + 2310400 = 4369^2 + 1520^2 .

It is a cyclic number.

It is not a de Polignac number, because 21398561 - 2² = 21398557 is a prime.

It is not a weakly prime, because it can be changed into another prime (21398569) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10699280 + 10699281.

It is an arithmetic number, because the mean of its divisors is an integer number (10699281).

Almost surely, 2^21398561 is an apocalyptic number.

It is an amenable number.

21398561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21398561 is an equidigital number, since it uses as much as digits as its factorization.

21398561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 12960, while the sum is 35.

The square root of 21398561 is about 4625.8578663854. The cubic root of 21398561 is about 277.6268867549.

Subtracting from 21398561 its reverse (16589312), we obtain a square (4809249 = 2193²).

The spelling of 21398561 in words is "twenty-one million, three hundred ninety-eight thousand, five hundred sixty-one".