1638781 has 4 divisors (see below), whose sum is σ = 1644064. Its totient is φ = 1633500.

The previous prime is 1638743. The next prime is 1638797. The reversal of 1638781 is 1878361.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 1878361 = 199 ⋅9439.

It is a 3-Lehmer number, since φ(1638781) divides (1638781-1)³.

It is a cyclic number.

It is not a de Polignac number, because 1638781 - 2⁷ = 1638653 is a prime.

It is a super-2 number, since 2×1638781² = 5371206331922, which contains 22 as substring.

It is an alternating number because its digits alternate between odd and even.

It is a Duffinian number.

1638781 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1638701) 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, 2145 + ... + 2806.

It is an arithmetic number, because the mean of its divisors is an integer number (411016).

2^1638781 is an apocalyptic number.

1638781 is the 573-rd centered decagonal number.

It is an amenable number.

1638781 is a deficient number, since it is larger than the sum of its proper divisors (5283).

1638781 is an equidigital number, since it uses as much as digits as its factorization.

1638781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5282.

The product of its digits is 8064, while the sum is 34.

The square root of 1638781 is about 1280.1488194737. The cubic root of 1638781 is about 117.8981453733.

The spelling of 1638781 in words is "one million, six hundred thirty-eight thousand, seven hundred eighty-one".