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Giuga numbers (A007850) are composite numbers \$n\$ such that, for each prime factor \$p_i\$ of \$n\$, \$p_i \mid \left( \frac n {p_i} -1 \right)\$. That is, that for each prime factor \$p_i\$, you can divide \$n\$ by the factor, decrement it and the result is divisible by \$p_i\$
For example, \$...