1645221891 has 4 divisors (see below), whose sum is σ = 2193629192. Its totient is φ = 1096814592.

The previous prime is 1645221841. The next prime is 1645221899. The reversal of 1645221891 is 1981225461.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1981225461 = 3 ⋅660408487.

It is not a de Polignac number, because 1645221891 - 2³⁰ = 571480067 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1645221899) 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, 274203646 + ... + 274203651.

It is an arithmetic number, because the mean of its divisors is an integer number (548407298).

Almost surely, 2^1645221891 is an apocalyptic number.

1645221891 is a deficient number, since it is larger than the sum of its proper divisors (548407301).

1645221891 is an equidigital number, since it uses as much as digits as its factorization.

1645221891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 548407300.

The product of its digits is 34560, while the sum is 39.

The square root of 1645221891 is about 40561.3349262570. The cubic root of 1645221891 is about 1180.5240157997.

The spelling of 1645221891 in words is "one billion, six hundred forty-five million, two hundred twenty-one thousand, eight hundred ninety-one".