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  "body": "\n## Summary\n\nThe order of a [[partial differential equation|GC5BDZ6QBC]] is the highest order of a partial derivative of the unknown function that occurs in it.\nIt is the first number used to sort PDEs, and it controls how many initial or boundary conditions a well-posed problem needs.\nSee the [[definition|YFFEKD37RP]].\n\n## Explanations\n\n### L1 human\n\nCount how many times the unknown is differentiated in each term, and take the largest count.\nThe [[transport equation|PVD7VZJZ9B]] $\\partial_t u + \\partial_x u = 0$ has order $1$.\nThe [[wave equation|5EHD2XBQDG]] $\\partial_t^2 u = \\partial_x^2 u$ has order $2$.\nA mixed derivative such as $\\partial_x \\partial_y u$ also counts as order $2$.\n",
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