It is a happy number.
It is a semiprime because it is the product of two primes.
It is a cyclic number.
It is a de Polignac number, because none of the positive numbers 2k-21671 is a prime.
It is a Duffinian number.
It is a congruent number.
221671 is an apocalyptic number.
21671 is a wasteful number, since it uses less digits than its factorization.
21671 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 1680.
The square root of 21671 is about 147.2107333043. The cubic root of 21671 is about 27.8800137777.
The spelling of 21671 in words is "twenty-one thousand, six hundred seventy-one".