1
×
2
is not divisible by
1
+
2
.
1
×
2
×
3
×
4
is not divisible by
1
+
2
+
3
+
4
.
1
×
2
×
3
×
4
×
5
×
6
is not divisible by
1
+
2
+
3
+
4
+
5
+
6
.
Is it true that there are infinitely many even positive integers m such that 1 × 2 × ⋯ × m is not divisible by 1 + 2 + ⋯ + m ?
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1 + 2 + ⋯ + k 1 × 2 × 3 × ⋯ × k = k + 1 2 ( k − 1 ) ! .Note that k is an even number so k − 1 and k + 1 are odd integers where k + 1 > k − 1 which shows that ( k − 1 ) has no factor of k + 1 .
Further note that 2 and k + 1 are co-prime to each and difference between k − 1 and k + 1 is of 2 so easily we concluded that if k − 1 is prime then k + 1 will be an odd number and vice or both will be prime. Hence there are infintely many even numbers. m
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1 + 2 + ⋯ + m 1 × 2 × ⋯ × m = 2 m ( m + 1 ) m ! = m + 1 2 ( m − 1 ) ! .
Now for any even positive integer m such that m + 1 is prime, 2 ( m − 1 ) ! will not have m + 1 as a term in its prime factorization, i.e., will not be divisible by m + 1 . So since there are an infinite number of primes, there will be infinitely many such integers m .