8 3 ≡ 7 5 n ( m o d 3 2 )
What is the least possible positive integer n satisfying the congruence above?
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very comprehensive answer.
how did you go from the first step to the second step?
If you mean literally the first two lines, then he applied mod 32 to each of the members: 83 ≡ 19 (mod 32) and 75 ≡ 11 (mod 32).
6th line, the last number (last remainder) should be a 1, I suppose
From the statement, 8 3 ≡ 7 5 n (mod 32), we can derive:
{ 8 3 m o d 3 2 = r 7 5 m o d 3 2 = r ⇒ { 8 3 = 3 2 k + r 7 5 = 3 2 l + r with l, r some positive integers ⇒ 8 3 − 3 2 k = 7 5 n − 3 2 l
And thus we obtain that: 7 5 n + 3 2 ( k − l ) = 8 3 . We see that g c d ( 7 5 , 3 2 ) = 1 . Because 1 ∣ 8 3 we can try solving the simpler problem:
7 5 x ∗ + 3 2 y ∗ = 1 x ∗ , y ∗ ∈ Z
Using the Euclidean algorithm we can write:
7 5 = 2 ∗ 3 2 + 1 1 3 2 = 2 ∗ 1 1 + 1 0 1 1 = 1 0 ∗ 1 + 1 1 1 = 7 5 − 2 ∗ 3 2 1 0 = 3 2 − 2 ∗ 1 1 1 = 1 1 − 1 0 1 = 1 1 − 1 0 = 1 1 − ( 3 2 − 2 ∗ 1 1 ) = 3 ∗ 1 1 − 3 2 = 3 ∗ ( 7 5 − 2 ∗ 3 2 ) − 3 2 = 3 ∗ 7 5 + ( − 7 ) ∗ 3 2
So a potential solution is: n = 3 ∗ 8 3 = 2 4 9 , k − l = − 7 ∗ 8 3 = − 5 8 1 . Using the general solutions formula we get:
( 2 4 9 + 3 2 m , − 5 8 1 − 7 5 m )
We can see: The smallest possible n is for m = − 7 ⇒ n = 2 5
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8 3 ≡ 7 5 n ( m o d 3 2 )
1 9 ≡ 1 1 n ( m o d 3 2 )
Then 1 9 = 1 1 n + 3 2 m for some integers n , m .
By using Euclidean Algorithm , we can solve this linear diophantine equation:
3 2 = 1 1 ⋅ 2 + 1 0
1 1 = 1 1 ⋅ 1 + 1 0
Thus, 1 = 1 1 − 1 0 = 1 1 − ( 3 2 − 1 1 ⋅ 2 ) = 3 ⋅ 1 1 − 3 2 .
Then 1 9 = 5 7 ⋅ 1 1 − 1 9 ⋅ 3 2 .
The initial solution for n is 5 7 , and since g c d ( 1 1 , 3 2 ) = 1 , n + 3 2 p will also be the solutions where p is some integer.
Since 5 7 ≡ 2 5 ( m o d 3 2 ) . The least possible n is 2 5 .
Checking for solutions: 7 5 ⋅ 2 5 ≡ 1 8 7 5 ≡ 1 8 5 6 + 1 9 ≡ 5 8 ⋅ 3 2 + 1 9 ≡ 1 9 ≡ 8 3 ( m o d 3 2 ) .