21661891 has 2 divisors, whose sum is σ = 21661892. Its totient is φ = 21661890.

The previous prime is 21661873. The next prime is 21661903. The reversal of 21661891 is 19816612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21661891 - 2¹¹ = 21659843 is a prime.

It is a plaindrome in base 11.

It is not a weakly prime, because it can be changed into another prime (21661831) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10830945 + 10830946.

It is an arithmetic number, because the mean of its divisors is an integer number (10830946).

Almost surely, 2^21661891 is an apocalyptic number.

21661891 is a deficient number, since it is larger than the sum of its proper divisors (1).

21661891 is an equidigital number, since it uses as much as digits as its factorization.

21661891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5184, while the sum is 34.

The square root of 21661891 is about 4654.2336640955. The cubic root of 21661891 is about 278.7610694928.

The spelling of 21661891 in words is "twenty-one million, six hundred sixty-one thousand, eight hundred ninety-one".