P and C:--

Find the number of positive integers 'm' such that (m-1)! is NOT exactly divisible by m excluding the prime values of 'm'.

2 4 0 1

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2 solutions

Jared Low
Jan 2, 2015

For positive non-primes m 4 m \leq 4 , we have 1 ( 1 1 ) ! = 1 , 4 ( 4 1 ) ! = 6 1\mid (1-1)!=1, 4\nmid(4-1)!=6 . So the only such non-prime value 4 \leq 4 fulfilling the conditions in the question is 4 4 .

For non-primes m > 4 m > 4 , we can express m m as a product m = d 1 d 2 m=d_1*d_2 , where d 1 , d 2 d_1, d_2 are distinct proper factors of m m (proper factors being factors of the number not equal to 1 or the number itself). We have d 1 , d 2 d_1, d_2 being factors of ( m 1 ) ! (m-1)! , since d 1 , d 2 m 1 d_1,d_2\leq m-1 and since the two are distinct, they are two non-repeating terms in the product ( m 1 ) ! = 1 × 2 × × ( m 1 ) (m-1)!=1 \times 2 \times \dots \times (m-1) , hence we have m = d 1 d 2 ( m 1 ) ! m=d_1*d_2 \mid (m-1)! for all non-primes m > 4 m>4

In conclusion, the only non-primes m m that satisfy this problem's conditions are m = 4 m=4 , consequently the number of such non-primes is 1 \boxed{1}

(m-1)! = (m-1)(m-2)(m-3)(m-4)......................3.2.1 For all prime values of m and 4, (m-1)! is not divisible by 'm' as for 'm'=4,,,3!/4=3/2 i.e it is not exactly divisible. Hence the answer is '1'.

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