1781 has 4 divisors (see below), whose sum is σ = 1932.
Its totient is φ = 1632.
The previous prime is 1777. The next prime is 1783. The reversal of 1781 is 1871.
It is a semiprime because it is the product of two primes.
It can be written as a sum of positive squares in 2 ways, for example, as 625 + 1156 = 25^2 + 34^2
It is a cyclic number.
It is not a de Polignac number, because 1781 - 22 = 1777 is a prime.
It is a super-2 number, since 2×17812 = 6343922, which contains 22 as substring.
It is a Duffinian number.
It is a plaindrome in base 9 and base 11.
It is a nialpdrome in base 13.
It is a congruent number.
It is not an unprimeable number, because it can be changed into a prime (1783) 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, 56 + ... + 81.
It is an arithmetic number, because the mean of its divisors is an integer number (483).
It is an amenable number.
1781 is a deficient number, since it is larger than the sum of its proper divisors (151).
1781 is a wasteful number, since it uses less digits than its factorization.
1781 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 150.
The product of its digits is 56, while the sum is 17.
The square root of 1781 is about 42.2018956920.
The cubic root of 1781 is about 12.1214518261.
Subtracting from 1781 its sum of digits (17), we obtain a square (1764 = 422).
The spelling of 1781 in words is "one thousand, seven hundred eighty-one".