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In some PDE books, ray is always used. For example, solve the PDE in the ray $x>0$. What does the ray means? points $(x,y)$ where $x$ is greater than zero?

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1 Answer

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This is one of the most basic topics of geometry . You have $3$ times of $1$-dimensional figures :- A line, A line-segment and A ray .

$(i)$ A line is a set of points in a straight path that extends in opposite directions infinitely .

$(ii)$ A line-segment is a part of a line which has a definite length and has $2$ endpoints .

$(iii)$ A ray is a part of a line that has $1$ endpoint and extends infinitely in $1$ direction .

We denote lines by $2$ arrows , rays with $1$ arrow and $1$ point , and line segments with $2$ points respectively .

Here is a figure to understand :-

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