Determine the number of solutions of the congruence x^{4}equiv61(mod 117)

Question
Congruence
asked 2020-12-25
Determine the number of solutions of the congruence \(x^{4}\equiv61(mod\ 117)\)

Answers (1)

2020-12-26
Step 1
Given: \(x^{4}\equiv61(mod\ 117)\)
\(117=3^{2}times 13\)
As \((phi(9))/(4,phi(9))=6/(4.6)=6/2=3 (here (4.6)denotes the g.c.d of(4.6))\)
and \((61)^{3}\equiv(-2)^{3}\equiv1(mod\ 9)\)
we deduce the congruence
\(x^{4}\equiv61(mod\ 9)\ has\ (4,\phi(9))=(4.6)=2\ solutions\)
Step 2
Similarity \(y\frac{\phi(13)}{4,\phi(13)}=\frac{12}{4.12}=\frac{12}{4}=3\)
and \((61)^{3}\equiv(-4)^{3}\equiv1(mod\ 13)\)
So, the congruence \(x^{4}\equiv61(mod\ 13)\ has\ (4,\phi(13))=(4.12)=4\ solutions\)
hence, the number of solutions of the congruence \(x^{4}\equiv61(mod\ 117)\ is\ 2\times 4=8\).
0

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