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  "body": "\n## Summary\n\nTrial division is the simplest method to factor an integer $n \u003e 1$ or to test whether it is prime.\nIt divides $n$ by the integers $d = 2, 3, 4, \\dots$ in turn, and it stops at the first $d$ that divides $n$, or when $d^2 \u003e n$.\nIf no $d$ with $d^2 \\le n$ divides $n$, then $n$ is prime.\n\n## Remarks\n\nThe method is correct because [[a composite number has a prime factor at most its square root|0EB6BBMQMC]].\nIt is enough to test the primes $d$, but a test of all integers $d$ is also correct.\nFor a number $n$ with $k$ digits, the method can need about $\\sqrt{n} \\approx 10^{k/2}$ divisions. Thus it is slow for large $n$.\n\n## Explanations\n\n### L1 human\n\nTo test whether $91$ is prime, try to divide it by $2$, $3$, $5$ and $7$. You can stop after $9$, because $10^2 \u003e 91$.\nThe division by $7$ works: $91 = 7 \\cdot 13$. So $91$ is not prime.\n",
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