Amicable numbers refer to a pair of natural numbers that have a special relationship in number theory, a branch of mathematics.

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In Japan, 4 and 9 are considered unlucky numbers.

This factory strictly controls the emission of hydrogen sulphide.

I saw a Hachijō blue rock thrush on Hachijō Island.

He turned fifty-six years old.

Isoroku is my best friend and a very kind person.

He understands the meaning of the proverb 'Teaching is half learning', which means that teaching leads to one's own learning.

By regularly checking the key performance indicators, you can understand the performance of the company and devise improvement measures.

Hydrogen is the most abundant element in the universe.