21316661 has 2 divisors, whose sum is σ = 21316662. Its totient is φ = 21316660.

The previous prime is 21316643. The next prime is 21316663. The reversal of 21316661 is 16661312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18447025 + 2869636 = 4295^2 + 1694^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21316661 is a prime.

It is a Sophie Germain prime.

Together with 21316663, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 21316661.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21316663) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10658330 + 10658331.

It is an arithmetic number, because the mean of its divisors is an integer number (10658331).

Almost surely, 2^21316661 is an apocalyptic number.

It is an amenable number.

21316661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21316661 is an equidigital number, since it uses as much as digits as its factorization.

21316661 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1296, while the sum is 26.

The square root of 21316661 is about 4616.9969677270. The cubic root of 21316661 is about 277.2722412214.

Adding to 21316661 its reverse (16661312), we get a palindrome (37977973).

The spelling of 21316661 in words is "twenty-one million, three hundred sixteen thousand, six hundred sixty-one".