9
$\begingroup$

I have these numbers and I couldn't guess the pattern of this question it might be easy or it might be hard whatever these are the numbers $$ f(43) = 13\\ f(79) = 40\\ f(111) = 120\\f(138)=161\\f(169) = 247\\f(256) = ??? $$ I tried to sum digits of the number but get nothing can any one help /:

$\endgroup$
3
  • 5
    $\begingroup$ Can you tell us a bit about where this puzzle comes from? $\endgroup$
    – Gareth McCaughan
    Commented Nov 28, 2017 at 16:42
  • 14
    $\begingroup$ There are an infinite number of functions that will agree at the 5 points given but have different values at 256. $\endgroup$
    – chepner
    Commented Nov 28, 2017 at 21:44
  • 3
    $\begingroup$ @chepner This objection applies to any "what's next in this sequence" question. Please move along. $\endgroup$ Commented Nov 29, 2017 at 9:26

3 Answers 3

29
$\begingroup$

The answer is:

619

Reason: f(x*10 + y) = … where x: N, y: [0, 9]

Because: $f(x*10+y) = x^2 - y$ $$f(43) = 4^2 -3 = 13$$ $$f(79) = 7^2 - 9 = 40$$ $$f(111) = 11^2 - 1 = 120$$ $$f(138) = 13^2 - 8 = 161$$ So: $$f(256) = 25^2 - 6 = 619$$

$\endgroup$
2
  • $\begingroup$ Why not explain notation like f(x*10 + y) = … where x: N, y: [0, 9]? $\endgroup$
    – user28434
    Commented Nov 29, 2017 at 15:49
  • $\begingroup$ @user28434 As you wish $\endgroup$
    – rudra
    Commented Nov 29, 2017 at 16:00
15
$\begingroup$

@rudha I am not allowed to make comments yet. I like your answer.

Since I am new to this notation style, I found the notation confusing because:

Using $f(xy)$ notation, $f(111)$ is ambiguous to me, as $x$ could be $1$, and $y$ could be $11$, such that $f(111)$ could also become $1^2 - 11$ resulting in $-10$. But, even before that, I thought $xy$ meant $x \times y$.

So, I just wanted to reformat it in the imperative style I am used to:

$$ f(xy) = x^2 - y $$ becomes: $$ f(z) = \lfloor z/10 \rfloor^2 - (z\bmod10)\,,$$ which becomes, in pseudocode: $$ f(z) = \mathrm{power}( \mathrm{floor}(z/10) , 2) - ( \mathrm{modulus}(z,10) ) $$

So, in JavaScript (which is easy to test in web browsers) it would be:

function f(z) { return Math.pow( Math.floor(z/10), 2) - z%10; }

$\endgroup$
1
  • $\begingroup$ It could also be shown as f(x \Vert y), which produces $f(x \Vert y)$, as long as you clarify the meaning. $\Vert$ is a symbol commonly used for digit concatenation, per Wolfram Mathworld and Math.SE. (I do like showing the explicit formula for a single number $z$, but I think this conveys @rudra's intention better.) $\endgroup$ Commented Nov 28, 2017 at 22:11
5
$\begingroup$

I believe the answer is:

3166

Reasoning:

By calculating the quartic regression equation (See Wolframalpha.com) the number 256 can be substituted into the equation yielding f(256)= 3166

$\endgroup$
2
  • 2
    $\begingroup$ Welcome to Puzzling! (Take the Tour!) To be a puzzle and not an "identify the mathematical sequence" question, this would have to be something that goes beyond mechanically fitting a formula to the literal numbers and then grinding out the next entry (see, for example, our guidance to number sequence posters). See other answers which would be more in the spirit of what to expect, and see if you can come up with a reasonable but interesting pattern! $\endgroup$
    – Rubio
    Commented Nov 28, 2017 at 23:44
  • $\begingroup$ btw it's wolframalpha not wolframalfa $\endgroup$
    – MCMastery
    Commented Nov 29, 2017 at 17:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.