31920821 has 2 divisors, whose sum is σ = 31920822. Its totient is φ = 31920820.

The previous prime is 31920809. The next prime is 31920839. The reversal of 31920821 is 12802913.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 31798321 + 122500 = 5639^2 + 350^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-31920821 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (31920841) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 15960410 + 15960411.

It is an arithmetic number, because the mean of its divisors is an integer number (15960411).

Almost surely, 2^31920821 is an apocalyptic number.

It is an amenable number.

31920821 is a deficient number, since it is larger than the sum of its proper divisors (1).

31920821 is an equidigital number, since it uses as much as digits as its factorization.

31920821 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 864, while the sum is 26.

The square root of 31920821 is about 5649.8514139754. The cubic root of 31920821 is about 317.2181424011.

The spelling of 31920821 in words is "thirty-one million, nine hundred twenty thousand, eight hundred twenty-one".