21640561 has 2 divisors, whose sum is σ = 21640562. Its totient is φ = 21640560.

The previous prime is 21640559. The next prime is 21640607. The reversal of 21640561 is 16504612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18011536 + 3629025 = 4244^2 + 1905^2 .

It is a cyclic number.

It is not a de Polignac number, because 21640561 - 2¹ = 21640559 is a prime.

Together with 21640559, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21640501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10820280 + 10820281.

It is an arithmetic number, because the mean of its divisors is an integer number (10820281).

Almost surely, 2^21640561 is an apocalyptic number.

It is an amenable number.

21640561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21640561 is an equidigital number, since it uses as much as digits as its factorization.

21640561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1440, while the sum is 25.

The square root of 21640561 is about 4651.9416376391. The cubic root of 21640561 is about 278.6695427603.

It can be divided in two parts, 21640 and 561, that added together give a square (22201 = 149²).

The spelling of 21640561 in words is "twenty-one million, six hundred forty thousand, five hundred sixty-one".