Find the minimum positive integer value of n such that 2 2 0 1 4 divides 2 0 1 3 n − 1 .
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This problem looks scary, but it's almost immediate once we write it out. Let 2 0 1 3 = 4 k + 1 , where k is odd. We want
2 2 0 1 4 ∣ ( 4 k + 1 ) n − 1
By considering the binomial expansion, we have:
2 2 0 1 4 ∣ ( 4 k ) n + ( n − 1 n ) ( 4 k ) n − 1 + … + ( 2 n ) ( 4 k ) 2 + ( 1 n ) ( 4 k ) 1
Observe that since
k
is odd, if
n
is odd, then the last term is divisible by 4 not 8, while all of the other terms are divisible by 16. Hence, we must have
n
=
2
n
1
.
Again, if
n
1
is odd, then the last term is divisible by 8 not 16, while all of the other terms are divisible by 32. Hence,
n
1
=
2
n
2
.
Again, if
n
2
is odd, then the last term is divisible by 16 not 32, while all of the other terms are divisible by 64. Hence
n
2
=
2
n
3
.
We can continually repeat this process, until we get "If
n
2
0
1
2
is odd, then the last term is divisible by
2
2
0
1
4
(but not
2
2
0
1
5
). Thus tells us that we must have
n
=
2
2
0
1
2
×
n
2
0
1
2
.
Hence, the smallest positive integer is n = 2 2 0 1 2 .
Note: I decided to present a basic approach to this problem. Yes there are shorter approaches which use higher-power theorems like lifting the exponent .
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Let n = 2 k × t , k , t are whole numbers, and t is odd.
We have 2 0 1 3 n − 1 = ( 2 0 1 3 2 k ) t − 1 = ( 2 0 1 3 2 k − 1 ) [ ( 2 0 1 3 2 k ) t − 1 + . . . + 2 0 1 3 2 k + 1 ]
Since t is odd, 2 0 1 3 n − 1 ⋮ 2 2 0 1 4 ⟺ 2 0 1 3 2 k − 1 ⋮ 2 2 0 1 4
Also, 2 0 1 3 2 k − 1 = ( 2 0 1 3 2 − 1 ) ( 2 0 1 3 2 + 1 ) ( 2 0 1 3 4 + 1 ) . . . ( 2 0 1 3 2 k − 1 + 1 )
Since 2 0 1 3 ≡ 1 ( m o d 4 ) ⟹ 2 0 1 3 2 i + 1 ≡ 2 ( m o d 4 )
Hence 2 0 1 3 n ⋮ 2 2 0 1 4 ⟺ ( k − 1 ) + 3 ≥ 2 0 1 4
Therefore, the minimum value of n is 2 2 0 1 2