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The docs say:
Function: gamma_incomplete (a, z)
The incomplete upper gamma function A&S 6.5.2:
Actually, A&S 6.5.2 is the incomplete lower gamma function, and 6.5.3 is the incomplete upper gamma function.
The online html pages probably need to be updated. From the command-line, ? gamma_incomplete says A&S 6.5.3.