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$Version (* "12.2.0 for Mac OS X x86 (64-bit) (December 12, 2020)" *) Clear["Global`*"] B = Erfc[x] Exp[-x^2/2] + Sqrt[2] Erfc[x/Sqrt[2]] Exp[-x^2]; If you explicitly state that a is a real value, Mathematica will tell you that the result is conditional on a > 0 BI = Assuming[Element[a, Reals], Integrate[B, {x, a, ∞}]] (* ...


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