661407721 has 4 divisors (see below), whose sum is σ = 712285252. Its totient is φ = 610530192.

The previous prime is 661407713. The next prime is 661407739. The reversal of 661407721 is 127704166.

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: 127704166 = 2 ⋅63852083.

It can be written as a sum of positive squares in 2 ways, for example, as 203918400 + 457489321 = 14280^2 + 21389^2 .

It is a cyclic number.

It is not a de Polignac number, because 661407721 - 2³ = 661407713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (661407701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 25438746 + ... + 25438771.

It is an arithmetic number, because the mean of its divisors is an integer number (178071313).

Almost surely, 2^661407721 is an apocalyptic number.

It is an amenable number.

661407721 is a deficient number, since it is larger than the sum of its proper divisors (50877531).

661407721 is a wasteful number, since it uses less digits than its factorization.

661407721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 50877530.

The product of its (nonzero) digits is 14112, while the sum is 34.

The square root of 661407721 is about 25717.8482964652. The cubic root of 661407721 is about 871.2773419797.

The spelling of 661407721 in words is "six hundred sixty-one million, four hundred seven thousand, seven hundred twenty-one".