Adding to 6521 its reverse (1256), we get a palindrome (7777).
6521 is nontrivially palindromic in base 6.
It is a strong prime.
It is a cyclic number.
It is a de Polignac number, because none of the positive numbers 2k-6521 is a prime.
It is a Sophie Germain prime.
It is a Chen prime.
It is an alternating number because its digits alternate between even and odd.
It is a Curzon number.
It is a plaindrome in base 11.
It is a nialpdrome in base 10.
It is a good prime.
26521 is an apocalyptic number.
It is an amenable number.
6521 is an equidigital number, since it uses as much as digits as its factorization.
6521 is an evil number, because the sum of its binary digits is even.
The square root of 6521 is about 80.7527089329. The cubic root of 6521 is about 18.6826323157.
The spelling of 6521 in words is "six thousand, five hundred twenty-one".