Determine existence in recursive working table
I'm not sure I fully understand what you expect to happen with the SQL you pasted .. (add up the numbers from 1-99 ignoring all even values?)  But this is a SQL to get around the syntax error you have:

WITH RECURSIVE t(n) AS (
    VALUES (1)
  UNION ALL
    SELECT n+1 FROM t WHERE n+1 not in (t.n) and n &lt; 100
)
SELECT sum(n) FROM t;


I would like to point out that the result of my SQL above returns 5050 because the "n+1" is never actually in the result set...  (You'd need to compare for n-1 in the result set if you wanted to ignore even numbers..)