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: 1461 = 3 ⋅487.
It is an Ulam number.
It is a D-number.
It is the 41-st Hogben number.
It is a Duffinian number.
1641 is a lucky number.
It is a plaindrome in base 6, base 9 and base 16.
It is a nialpdrome in base 13 and base 14.
It is a zygodrome in base 6.
It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 1641.
It is an amenable number.
1641 is an equidigital number, since it uses as much as digits as its factorization.
1641 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 550.
The square root of 1641 is about 40.5092582011. The cubic root of 1641 is about 11.7951334907.
Adding to 1641 its sum of digits (12), we get a triangular number (1653 = T57).
The spelling of 1641 in words is "one thousand, six hundred forty-one".