2191721 has 6 divisors (see below), whose sum is σ = 2549610. Its totient is φ = 1878576.

The previous prime is 2191711. The next prime is 2191723. The reversal of 2191721 is 1271912.

It can be written as a sum of positive squares in only one way, i.e., 2059225 + 132496 = 1435^2 + 364^2 .

It is a de Polignac number, because none of the positive numbers 2^k-2191721 is a prime.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (2191723) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 22316 + ... + 22413.

It is an arithmetic number, because the mean of its divisors is an integer number (424935).

2^2191721 is an apocalyptic number.

It is an amenable number.

2191721 is a deficient number, since it is larger than the sum of its proper divisors (357889).

2191721 is an equidigital number, since it uses as much as digits as its factorization.

2191721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 44743 (or 44736 counting only the distinct ones).

The product of its digits is 252, while the sum is 23.

The square root of 2191721 is about 1480.4462165172. The cubic root of 2191721 is about 129.8957942048.

The spelling of 2191721 in words is "two million, one hundred ninety-one thousand, seven hundred twenty-one".