Win 10, Office 365

All I did was use Option Explicit and Dim z1 as a Double

Possibly (bold, underlined) not Dim-ing it on a Mac it'd be a variant and that might cause an issue

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Option Explicit


Sub a_test_digammad()
    Dim z1 As Double


    For z1 = 1 To 4.01 Step 0.1
        Debug.Print z1, diGammad(z1)
    Next z1
End Sub


Function diGammad(x)
    diGammad = x
End Function