6445

6445 is a natural number that follows 6444 and precedes 6446. In decimal notation it is written as 6445. The digits sum to 19, so it is not divisible by 3; it is odd. Because it ends in 5, it is divisible by 5, and 6445 = 5 × 1289. The number 1289 is prime, so 6445 is a semiprime (a product of two primes). The positive divisors of 6445 are 1, 5, 1289, and 6445.

In binary it is 1100100101101, and in hexadecimal it is 0x192D.

It is congruent to 1 modulo 9 (since the digit sum is 19) and congruent to 10 modulo 11. Euler's totient φ(6445) = (5−1)(1289−1) = 5152.

As a semiprime, 6445 is often cited in introductory number theory to illustrate factorization and primality concepts, providing a concrete example of a number that factors into two distinct primes.