add exp(-1) constant

useful in many situations, also -exp(-1) is the branching point of
Lambert's W_0(t) and W_{-1}(t)

Author: alyst

src/IrrationalConstants.jl

@@ -26,7 +26,8 @@ export
     logten,     # log(10)
     logπ,       # log(π)
     log2π,      # log(2π)
-    log4π       # log(4π)
+    log4π,      # log(4π)
+    invℯ        # 1 / ℯ
 
 include("stats.jl")
 

src/stats.jl

@@ -28,3 +28,5 @@
 @irrational logπ    1.1447298858494001741 log(big(π))
 @irrational log2π   1.8378770664093454836 log(2 * big(π))
 @irrational log4π   2.5310242469692907930 log(4 * big(π))
+
+@irrational invℯ    0.367879441171442321595 inv(big(ℯ))

test/runtests.jl

@@ -40,3 +40,6 @@ end
   @test isapprox(log(4pi), log4π)
 end
 
+@testset "1/e" begin
+  @test isapprox(invℯ, exp(-1))
+end