21810661 has 2 divisors, whose sum is σ = 21810662. Its totient is φ = 21810660.

The previous prime is 21810653. The next prime is 21810673. The reversal of 21810661 is 16601812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19088161 + 2722500 = 4369^2 + 1650^2 .

It is a cyclic number.

It is not a de Polignac number, because 21810661 - 2³ = 21810653 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21810361) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10905330 + 10905331.

It is an arithmetic number, because the mean of its divisors is an integer number (10905331).

Almost surely, 2^21810661 is an apocalyptic number.

It is an amenable number.

21810661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21810661 is an equidigital number, since it uses as much as digits as its factorization.

21810661 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 576, while the sum is 25.

The square root of 21810661 is about 4670.1885400913. The cubic root of 21810661 is about 279.3977745365.

The spelling of 21810661 in words is "twenty-one million, eight hundred ten thousand, six hundred sixty-one".