21661831 has 2 divisors, whose sum is σ = 21661832. Its totient is φ = 21661830.

The previous prime is 21661813. The next prime is 21661841. The reversal of 21661831 is 13816612.

Together with previous prime (21661813) it forms an Ormiston pair, because they use the same digits, order apart.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21661831 - 2¹⁵ = 21629063 is a prime.

It is a super-2 number, since 2×21661831² = 938469844545122, which contains 22 as substring.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21661831.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21661841) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10830915 + 10830916.

It is an arithmetic number, because the mean of its divisors is an integer number (10830916).

Almost surely, 2^21661831 is an apocalyptic number.

21661831 is a deficient number, since it is larger than the sum of its proper divisors (1).

21661831 is an equidigital number, since it uses as much as digits as its factorization.

21661831 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1728, while the sum is 28.

The square root of 21661831 is about 4654.2272183468. The cubic root of 21661831 is about 278.7608121179.

The spelling of 21661831 in words is "twenty-one million, six hundred sixty-one thousand, eight hundred thirty-one".