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 2020-10-21, 17:52 #1 petrw1 1976 Toyota Corona years forever!     "Wayne" Nov 2006 Saskatchewan, Canada 445910 Posts Can someone help me understand this... Or simplify it: How does such a seemingly odd string of irrational numbers equate to exactly 142? Better yet how would someone have come up with this --- mathematically; not via trial and error? Interestingly, the first half is almost exactly twice the second half. (√√7! + √√7!) * (√(√7! + (√7!)/7!)) = 142 Thanks
 2020-10-21, 18:25 #2 Viliam Furik   Jul 2018 Martin, Slovakia 111111002 Posts The whole process is here. If we say the 7 factorial is x, then we can generalize it for x, and by shifting things around, we find that expression of the form (√√x + √√x) * √(√x + (√x)/x) is a whole number whenever x+1 is a square.
2020-10-21, 18:48   #3
petrw1
1976 Toyota Corona years forever!

"Wayne"
Nov 2006

73×13 Posts

Quote:
 Originally Posted by Viliam Furik The whole process is here. If we say the 7 factorial is x, then we can generalize it for x, and by shifting things around, we find that expression of the form (√√x + √√x) * √(√x + (√x)/x) is a whole number whenever x+1 is a square.
Thanks a lot....no big deal, but you dropped he 2x on line 2 of your napkin.

So If I knew what I was doing I could reverse this process and get other whole numbers starting with √(x!+1) ??
x can be 4, 5, or 7.

Last fiddled with by petrw1 on 2020-10-21 at 18:54

2020-10-21, 19:26   #4
Viliam Furik

Jul 2018
Martin, Slovakia

111111002 Posts

Quote:
 Originally Posted by petrw1 Thanks a lot....no big deal, but you dropped he 2x on line 2 of your napkin.
Yes, I know, thus the implication arrow afterwards, instead of an equality sign. Removing the 2 doesn't change the rest. If I were to get some fraction in the end, I could simply put it back. But it doesn't change the rationality, which is the whole question.

And, it isn't a napkin. (But I think you know that) It is my notebook I use for maths in the school. (BTW, I am graduating in May 2021, at least that's what I thought a year ago. Who knows what else might Covid take - we are learning online since the last Monday, and it's possible it will be until Christmas)

Quote:
 Originally Posted by petrw1 So If I knew what I was doing I could reverse this process and get other whole numbers starting with √(x!+1) ?? x can be 4, 5, or 7.
Yes! Absolutely. You can shove in any number for x, even primorials, perfect powers, and also Riesel primes with even powers of base and square ks (based on few look-ups, they might not exist), but sadly enough, no Mersenne primes except M2, as 2p - 1 + 1 = 2p, which is not a square if the p is odd.

----
EDIT:
Silly me. Of course there can't be a Riesel prime with k being square and n being even, because of the almighty algebraic factors of a2 - 1 = (a-1)(a+1)

Last fiddled with by Viliam Furik on 2020-10-21 at 19:36

2020-10-22, 07:43   #5
LaurV
Romulan Interpreter

Jun 2011
Thailand

22×7×11×29 Posts

Quote:
 Originally Posted by petrw1 (√√7! + √√7!) * (√(√7! + (√7!)/7!)) = 142
Let $$\alpha=\sqrt{7!}$$. You have $$2\sqrt\alpha\cdot\sqrt{\alpha+\frac{\alpha}{\alpha^2}}$$. Which, when multiply the radicals and simplify the fraction under it, becomes $$2\sqrt{\alpha^2+1}$$. Now substitute back the $$\alpha$$, you have $$2\sqrt{7!+1}$$, or $$2\sqrt{5041}$$, which is 2*71.

 2020-10-22, 11:37 #6 Dr Sardonicus     Feb 2017 Nowhere 378410 Posts Yes, substituting x for 7! does make things much easier to handle. The obvious regrouping of the first part of the expression gives $2x^{\frac{1}{4}}\cdot$$x^{\frac{1}{2}}\;+\;x^{\frac{-1}{2}}$$^{\frac{1}{2}}$ The "obvious" multiplication then gives $2\sqrt{x+1}$. I note that things can go wrong for complex values of "x" that aren't positive real numbers.
 2020-10-22, 16:45 #7 petrw1 1976 Toyota Corona years forever!     "Wayne" Nov 2006 Saskatchewan, Canada 73·13 Posts Thanks all
 2020-10-27, 17:45 #8 petrw1 1976 Toyota Corona years forever!     "Wayne" Nov 2006 Saskatchewan, Canada 73×13 Posts I can follow the simplification that gets one to 2 x SQRT(7!+1) = 142 Can someone explain how you would start at 2 x SQRT(7!+1) = 142 and given that the final equation must contain only 7's, get to the original equation I supplied in post 1. For example the 2 at the front could become something like (7+7)/7 OR + iff that function can be divided out in another part of the entire formula to equal 2. ....confused??? Me too. (My son has a game/puzzle app when he must find a formula using the least number of each digit from 1 to 9 to get a number) He is limited to +. -, x, /, SQRT and concatenation. ie 77 or 777
 2020-10-28, 04:54 #9 LaurV Romulan Interpreter     Jun 2011 Thailand 22·7·11·29 Posts Do you mean like engineeers112?. (scroll through the ppt presentation there)
2020-10-28, 05:42   #10
petrw1
1976 Toyota Corona years forever!

"Wayne"
Nov 2006

73×13 Posts

Quote:
 Originally Posted by LaurV Do you mean like engineeers112?. (scroll through the ppt presentation there)
Exactly ... that is sooooo much clearer!!!!

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