31720021 has 4 divisors (see below), whose sum is σ = 31951692. Its totient is φ = 31488352.

The previous prime is 31719991. The next prime is 31720049. The reversal of 31720021 is 12002713.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12002713 = 83 ⋅144611.

It can be written as a sum of positive squares in 2 ways, for example, as 7365796 + 24354225 = 2714^2 + 4935^2 .

It is a cyclic number.

It is not a de Polignac number, because 31720021 - 2⁷ = 31719893 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 31719977 and 31720004.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (31720121) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 115630 + ... + 115903.

It is an arithmetic number, because the mean of its divisors is an integer number (7987923).

Almost surely, 2^31720021 is an apocalyptic number.

It is an amenable number.

31720021 is a deficient number, since it is larger than the sum of its proper divisors (231671).

31720021 is a wasteful number, since it uses less digits than its factorization.

31720021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 231670.

The product of its (nonzero) digits is 84, while the sum is 16.

The square root of 31720021 is about 5632.0530004608. The cubic root of 31720021 is about 316.5515823111.

Adding to 31720021 its reverse (12002713), we get a palindrome (43722734).

The spelling of 31720021 in words is "thirty-one million, seven hundred twenty thousand, twenty-one".