Remark 1
Tag 0AYH, permanent. Cite it as [Bourbaki, Tag 0AYH].
Lie Groups and Lie Algebras, chapter VI, § 4. Classification of root systems
The graph A_1 reduces to a single node; we denote it also by B_1 or C_1. The graph B_2 \circ-\circ-\circ-\circ is also denoted by C_2. The graph A_3 \circ-\cdots-\circ is also denoted by D_3. Finally, D_2 denotes the graph consisting of two unconnected nodes. (These conventions are derived from the properties of the corresponding root systems, cf. nos. 5 to 8.)
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.
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