I can argue that, given more precise question, a sort of tasks about sequences like "1,2,3,4,5,?" makes perfect sence.
If you're referring to set of functions such that 
f(1) = 1
f(2) = 2
f(3) = 3
f(4) = 4
f(5) = 5

[download]
and then calculate f(6) then indeed there surely exists infinite number of different f's conforming these conditions and yet f(6) equals to -7 x phi.

But when you limit answers to functions having only natural numbers, of special kind (let that be polynoms with integral coefficients, with the largest power *no* more than three, then one and only one function fulfills the condition, and this is f(x)=x, and the correct answer is 6.)

Like everywhere else in mathematics, very many things depend on exact phrasing.

addition but I agree that OP has very badly phrased task

update 2+1 small typos

In reply to Re^2: Reverse engineering a formula... by vkon
in thread Reverse engineering a formula... by devnul

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.