Arithmetic Expansion in shell.

[Glen Johnson <gjohnson%valcom.com@localhost>]

NetBSD-users,
I am trying to understand what I am doing wrong when using sh but not ksh.
In /bin/ksh I type:
echo $(( i=i+1 ))
I get the next value for i.

In /bin/sh ,  I type the same thing:
echo $(( i=i+1 ))
I get this;  arith: syntax error: " i=i+1 "

Here is an excerpt from man sh:
----8<----------------8<----
 Arithmetic Expansion
    Arithmetic expansion provides a mechanism for evaluating an arithmetic
    expression and substituting its value.  The format for arithmetic 
expan-
    sion is as follows:

          $((expression))
   ...

   Shell variables may be referenced by name inside an arithmetic 
expression, without needing a
    ``$'' sign.
----8<----------------8<----

So what is the syntax supposed to be for /bin/sh?

Thanks,
Glen