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sod(N) = pod(sqrt(N)) (Posted on 2023-02-09) Difficulty: 3 of 5
(a) Find two positive integers, N1 and N2, such that for each:
* the sum of digits of the integer equals the product of digits of its square root, and
* the digit '1' does not appear in either the original integer or its square root.

(b) Can you find any others?

(c) Can you explain why there would be so many more solutions if we allowed the digit '1' to appear?

No Solution Yet Submitted by Larry    
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Comments: ( Back to comment list | You must be logged in to post comments.)
re: thought on (c) Comment 3 of 3 |
(In reply to thought on (c) by Charlie)

Allowing 1's in only the sum of the original integer adds only 3 new solutions:


     N        sqrt(N)    sod(N)   pod(sqrt(N))
   81    9    9    9
   1296     36     18     18
   8649     93     27     27
   110889      333       27       27
   27269284       5222         40         40

But allowing 1's in the pod of the square root adds a lot:

   8649     93     27     27
   3663396      1914        36        36
   4562496      2136        36        36
   5363856      2316        36        36
   26347689       5133         45         45
   27269284       5222         40         40
   28227969       5313         45         45
   84879369       9213         54         54
   228553924       15118          40          40
   299739969       17313          63          63
   455096889       21333          54          54
   543495969       23313          54          54
   26605850769        163113            54            54
   36524943225        191115            45            45
   36907788996        192114            72            72
   45502009344        213312            36            36
   398428588944         631212             72             72
   658389433744         811412             64             64
   662778348544         814112             64             64
   707469396544         841112             64             64
   3280854783969         1811313              72              72
   3287378750769         1813113              72              72
   3690709738884         1921122              72              72


Such allowing is in fact the secret to 1's expansion: it prevents the greater power of multiplication to outstrip the power of addition, as multiplication causes an exponential increase, vs addition's arithmetic increase. Even 2^10 exceeds 9*10.



  Posted by Charlie on 2023-02-09 09:55:06
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