How to code "e" by itself in equation e-1?

How do I code for an equation just using e-1?

[Torsten]

e is coded as exp(1)

Answer 1

[Matt J]

e=exp(+1)

Answer 2

[Steven Lord]

@Torsten and @Matt J are correct to suggest the exp function, but if the quantity x/a can be very small you might also want to look at the expm1 function.