645244781 has 4 divisors (see below), whose sum is σ = 645439452. Its totient is φ = 645050112.

The previous prime is 645244771. The next prime is 645244837. The reversal of 645244781 is 187442546.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 187442546 = 2 ⋅93721273.

It can be written as a sum of positive squares in 2 ways, for example, as 487968100 + 157276681 = 22090^2 + 12541^2 .

It is a cyclic number.

It is not a de Polignac number, because 645244781 - 2¹⁰ = 645243757 is a prime.

It is a super-2 number, since 2×645244781² = 832681654815475922, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (645244771) 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, 92276 + ... + 99021.

It is an arithmetic number, because the mean of its divisors is an integer number (161359863).

Almost surely, 2^645244781 is an apocalyptic number.

It is an amenable number.

645244781 is a deficient number, since it is larger than the sum of its proper divisors (194671).

645244781 is a wasteful number, since it uses less digits than its factorization.

645244781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 194670.

The product of its digits is 215040, while the sum is 41.

The square root of 645244781 is about 25401.6688624980. The cubic root of 645244781 is about 864.1215450612.

The spelling of 645244781 in words is "six hundred forty-five million, two hundred forty-four thousand, seven hundred eighty-one".