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Let $n=pq$ product of two different primes. Let $d=\gcd(p-1,q-1)$. Prove that $n$ is a Fermat pseudoprime to the base $a$ if and only if $a^d=1 \mod n$

I believe that I get: if $a^d=1 \mod n$ then $n$ is a pseudoprime to the base $a$.

But I can not get the other implication.

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Because $pq$ is a pseudoprime, we have $a^{pq-1} \equiv 1 \mod pq$.

By Fermat's little theorem, $a^{q-1} \equiv 1 \mod q$.

Combining this with $a^{pq-1} \equiv 1 \mod q$, we get $a^{p-1} \equiv 1 \mod q$.

Therefore, $a^{\gcd(p-1,q-1)} \equiv 1 \mod q$.

In the same way $a^{\gcd(p-1,q-1)} \equiv 1 \mod p$.

Hence we get $a^d = a^{\gcd(p-1,q-1)} \equiv 1 \mod pq$

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  • $\begingroup$ How do you know that $a^{pq-1}=1 mod(q)$? $\endgroup$ Apr 21 '17 at 15:20
  • $\begingroup$ $a^{pq-1} \equiv 1 \mod pq$, because it is a pseudoprime. Hence also $a^{pq-1} \equiv 1 \mod q$. $\endgroup$
    – wythagoras
    Apr 21 '17 at 15:21
  • $\begingroup$ Later... $a^{gcd(p-1,q-1)}=1 mod q$, why? That power is less than $p-1$ and $q-1$ $\endgroup$ Apr 21 '17 at 15:33
  • $\begingroup$ If $a^b \equiv 1 \mod k$ and $a^c \equiv 1 \mod k$, then $a^{\gcd(b,c)} \equiv 1 \mod k$. $\endgroup$
    – wythagoras
    Apr 21 '17 at 15:35
  • $\begingroup$ Uhmmm thank you very much! $\endgroup$ Apr 21 '17 at 15:39

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