2157721 has 2 divisors, whose sum is σ = 2157722. Its totient is φ = 2157720.

The previous prime is 2157709. The next prime is 2157731. The reversal of 2157721 is 1277512.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 2088025 + 69696 = 1445^2 + 264^2 .

It is a cyclic number.

It is not a de Polignac number, because 2157721 - 2⁹ = 2157209 is a prime.

2157721 is a lucky number.

It is not a weakly prime, because it can be changed into another prime (2157731) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 1078860 + 1078861.

It is an arithmetic number, because the mean of its divisors is an integer number (1078861).

2^2157721 is an apocalyptic number.

It is an amenable number.

2157721 is a deficient number, since it is larger than the sum of its proper divisors (1).

2157721 is an equidigital number, since it uses as much as digits as its factorization.

2157721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 980, while the sum is 25.

The square root of 2157721 is about 1468.9183095053. The cubic root of 2157721 is about 129.2206028418.

The spelling of 2157721 in words is "two million, one hundred fifty-seven thousand, seven hundred twenty-one".