Solovay-Strassen test

Which of the following integers a a can be used to prove that 403 is composite using the Solovay-Strassen primality test ?

181 191 201 211

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1 solution

Andrius Gegužis
Jul 14, 2017

Let's check the numbers using Solovay-Strassen primality test .

Let's start with 201. The Solovay- Strassen primality test says that if ( a / n ) a n 1 2 ( m o d n ) (a/n)≢ a^\frac{n-1}{2}\ (mod n) , (a is an integer coprime to n), then n is a composite number. Now let's look how it works with 201: We have to compute 201^201 mod 403 because (403-1)/2 = 201.

( 201 ) 2 01 = ( ( 201 ) 3 ) ) 6 7 ( 151 ) 6 7 = 151 ( ( 151 ) 2 ) 3 3 151 ( 233 ) 3 3 = 151 ( ( 233 ) 3 ) 1 1 151 37 6 1 1 = 151 376 ( ( 376 ) 2 ) 5 356 ( 326 ) 5 = 356 ( 326 ) 3 ( 326 ) 2 356 66 287 = 6743352 356 ( m o d 403 ) (201)^201=((201)^3))^67 ≡(151)^67=151∙((151)^2 )^33 ≡151∙(233)^33=151∙((233)^3 )^11≡ 151∙ 376^11= 151∙376∙((376)^2)^5≡356∙(326)^5=356∙(326)^3∙(326)^2≡356∙66 ∙287=6743352 ≡356 (mod 403)

We got the remainder 356 and it can't be equal to Jacobi symbol. So the answer is 201 .

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