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Phi Proximity Poser (Posted on 2023-02-07) Difficulty: 3 of 5
Using each of the digits 1 to 9 exactly once, and the standard arithmetic operators, +,-, *, /, and, () as many times as required; come up with a value that is closest to φ.
This means, minimizing abs(V - φ), where V is the obtained value and, abs is the absolute function.

(I) Use only the individual digits. Concatenations like (98-34)/(1*2) - (56/7) are NOT allowed.

(II) Concatenations are allowed.

Note: φ is the golden ratio or the golden mean.

See The Solution Submitted by K Sengupta    
Rating: 5.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
computer exploration on part I | Comment 2 of 3 |
Not looking to spend a lot of run time awaiting an answer, I ran

clc,clearvars
global phi availOps stack availDigs bestForm bestVal form
phi=(1+sqrt(5))/2;
digSet=1:9;
availOps='+-*/';
stack=[];
availDigs=digSet;
bestVal=9999999;
form='';
findIt()

function findIt()
global phi availOps stack availDigs bestForm bestVal form 

for whDig=1:length(availDigs)
   dig=availDigs(whDig);
   availDigs(whDig)=[];
   stack=[stack dig];
   form=[form char(string(dig))];
   findIt();
   stack(length(stack))=[];
   form(length(form))=[];
   availDigs=[availDigs(1:whDig-1) dig availDigs(whDig:end)];
end
if length(stack)>1
  for whOp=1:length(availOps)
    Op=availOps(whOp);
    saveform=form;
    form=[form Op];
    if Op~='/' || stack(end)~=0
      v=eval([num2str(stack(end-1),16) Op num2str(stack(end),16)]);
      if abs(phi-v)<abs(phi-bestVal)
         bestVal=v;
         bestForm=form(length(stack)-1:end); % before stack reduction
         disp([v phi-v])
         disp(['     ' bestForm])
         disp(' ')
      end
      stacksave=stack;
      stack=[stack(1:end-2) v];
      if ~isempty(availDigs)
        findIt();
      end
      stack=stacksave;     
    end
    form=saveform;
  end
end

end

for a few minutes. It produces the RPN (Reverse Polish Notation) formula for the computation. It produces some results using only part of the set of 9 digits (e.g., (7+8)/9 was the algebraic equivalent of an early one), but the later, more accurate, ones used all nine.  The last one produced before I terminated the program gave the following:


          The approximation         How far off
          1.61801242236025      2.15663896463436e-05
          
      The RPN formula     
     1234569*8-/+*--7/
     
Converted to Algebraic form, that's 

(1-(2-(3*(4+5/(6*9-8)))))/7.

I also tried reversing the order of the attempted digits, and using random order several times, but this remained the best I found.

  Posted by Charlie on 2023-02-07 14:39:21
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