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Belonging (Posted on 2007-04-29) Difficulty: 4 of 5
In this short set of numbers two do not belong:
1, 16, 64, 512, 4096, 46656, 1000000, 2985984

Why?

The numbers which belong have two special properties. Those not conforming have one of these which is not the same for each of them.

  Submitted by brianjn    
Rating: 3.6667 (3 votes)
Solution: (Hide)
The numbers can be seen as squares or cubes however some are both, and they stay.

16 is the the square of 4.
512 is the cube of 8.

Each of the other numbers can be represented as both a square and a cube:

      1 = 1*1               1 = 1*1*1
     64 = 8*8              64 = 4*4*4
   4096 = 64*64          4096 = 16*16*16
  46656 = 216*216       46656 = 36*36*36 
1000000 = 1000*1000   1000000 = 100*100*100
2985984 = 1728*1728   2985984 = 144*144*144

--------------------------------------------------
Note 1:
The set was derived from a sequence of numbers which had the property of being square as well as being cube, see 3rd column in table below. Finding a square and cube which were neither of the other was easy.
Note 2:
The idea arose partly from Brian Smith's "Square Patterns" and something with which that I had been toying.

The thought basically was: "Can a given number exist as both a square and a cube?" Having some doubts (possibly created from reading "Fermat's Last Theorem") I constructed a table as below in a spreadsheet:

 A     A^3      (A^3)^2     (A*A)^3
 1      1          1           1
 2      8         64          64
 3     27        729         729
 4     64       4096        4096       
 5    125      15625       15625
 6    ......

The table was my source of data.
Note 2:
Charlie made a point about 6th root in a response to one comment.
The full series can be generated as x^6; x^(2*3), that to which Charlie referred.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
SolutionPuzzle AnswerK Sengupta2022-07-19 00:15:07
re: A simpler solutionbrianjn2007-05-01 08:56:28
A simpler solutionGuest2007-04-30 21:28:45
Some ThoughtsAn observation of isolation (with spoiler)Dej Mar2007-04-30 02:31:11
re: One Solution (spoiler?)Charlie2007-04-29 16:41:06
Revised commentsKenny M2007-04-29 16:38:43
One Solution (spoiler?)Kenny M2007-04-29 16:20:39
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