You misunderstood my point... f in this example is the function which computes the first number that does not show up in the first X samples of OEIS sequences(s

You misunderstood my point... f in this example is the function which computes the first number that does not show up in the first X samples of OEIS sequences(so in the paper cited, OEIS stores only the first 180, so when x is 180, f(x) = 20067). Interestingly, this will create a paradox because it could itself be expressed as an OEIS sequence, which would mean that it does not belong in the sequence... but if it doesn't belong, then it should belong(Russell's paradox anyone :D).