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An Octagonal Problem (Posted on 2006-06-13) Difficulty: 3 of 5
An octagonal number is a number that is represented by the set of figures given below.

A certain octagonal number is the sum of the squares of three terms in an arithmetic sequence with a common difference of 704. What is that octagonal number?

See The Solution Submitted by K Sengupta    
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Solution re: For starters -- computer solution (spoiler) Comment 2 of 2 |
(In reply to For starters by e.g.)

This UBASIC program uses e.g.'s expressions for the square sum and the octagonal numbers:

   10   for N=1 to 1000000
   20    V=3*N*N-2*N-2*704*704
   30    if V@3=0 and V>0 then
   40      :M=int(sqrt(V/3)+0.5)
   50      :if M*M=V/3 then
   60        :print M,3*m*m+2*704*704
   70   next

 2836    25119920

The result indicates the octagonal number is 25,119,920 and the middle term in the arithmetic sequence is 2,836.

25,119,920 is the 2,894-th octagonal number, based on the formula e.g. found.

  Posted by Charlie on 2006-06-13 11:07:39
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