645261691 has 4 divisors (see below), whose sum is σ = 645337000. Its totient is φ = 645186384.

The previous prime is 645261653. The next prime is 645261709. The reversal of 645261691 is 196162546.

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: 196162546 = 2 ⋅98081273.

It is a cyclic number.

It is not a de Polignac number, because 645261691 - 2⁹ = 645261179 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (645261191) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 22866 + ... + 42583.

It is an arithmetic number, because the mean of its divisors is an integer number (161334250).

Almost surely, 2^645261691 is an apocalyptic number.

645261691 is a deficient number, since it is larger than the sum of its proper divisors (75309).

645261691 is an equidigital number, since it uses as much as digits as its factorization.

645261691 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 75308.

The product of its digits is 77760, while the sum is 40.

The square root of 645261691 is about 25402.0017124635. The cubic root of 645261691 is about 864.1290937043.

The spelling of 645261691 in words is "six hundred forty-five million, two hundred sixty-one thousand, six hundred ninety-one".