## is the kummer function monotonic decreasing in first argument?

Let $b >0$ and $x<0.$ If $a_1 < a_2 < 0$ are integers then ${}_1F_1(a_2;b;x) < {}_1F_1(a_1;b;x).$ Is it true that in fact ${}_1F_1(a;b;x)$ is monotonic decreasing as a function of $a$?

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