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QUESTION
Let $n=q_1q_2\ldots q_k$ where the $q_i$ are distinct primes and
$k>1$. Show that if $n$ is a Carmichael number then $q_i-1|n-1$
for each $i$. (This is the converse of the result you proved in
example sheet 4, no. 5). Hence show that there is no Carmichael
number of the form $3.5.q$, where $q$ is any prime $>5$.
ANSWER
Suppose $n$ is a Carmichael number. The, for any $b$ satisfying
gcd$(b,n)=1$, we have $b^{n-1}\equiv1$ mod $n$, Now $q_i$ is
prime, so we can find a primitive element $g_i$ say mod $q_i$. The
$q_i$ are distinct, so the Chinese Remainder Theorem allows us to
find a unique solution mod $n$ to the simultaneous congruences
$x\equiv g_i$ mod $q_i$ for $1\leq i\leq n$. Let $b$ be this
unique solution. The gcd$(b,n)=1$ since gcd$(b,q_i)=1$ for each
$i$. Thus $b^{n-1}\equiv1$ mod $n$, and so $b^{n-1}\equiv1$ mod
$q_i$ for each $i$. But $b\equiv g_i$ mod $q_i$, and $g_i$ has
order $q_i-1$ mod $q_i$ as $g_i$ is a primitive element mod $q_i$.
Thus $q_i-1|n-1$, and this is true for each $i$, as required.
Now suppose $n=3.5.q$ is a Carmichael number, where $q$ is a prime
$>5$, By the above, $n$ is divisible by 2,4 and $q-1$. Set
$q-1=t$. Then $n=15(t+1)$, so $n-1=15t+14$. Since $t|n-1$ we have
$t|14$. Thus $t$=1,2,7 or 14, which makes $q=t+1$ equal to 2,3,8
or 25, none of which is a prime $>5$. Thus no such Carmichael
number exists.
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