"How did you get U'_(k+2)=U''_(k+2)? "
Cf. (20°) in doc: Revista Foaie Matematică: www.fmatem.moldnet.md/1_(v_sor_05).htm
in pdf:
http://fox.ivlim.ru/docs/sorokine/flt.pdf
"How did you get U'_{k+3}=U''_{k+3}= v > 0?"
1) u > 0, hence:
2) u_{k+1} = "9" > 0, hence:
3) u_{k+2} = 2 > 0, hence:
4) v = (a_{k+2} + b_{k+2} – c_{k+2})_1 = (1; 2) > 0; hence:
5) U''_{k+3} = (a_{k+2} + b_{k+2} – c_{k+2})_1 = v > 0.
6) U' > 0 since u_{k+1} = "9" = n – 1; hence:
7) U'_{k+3} > 0.
8) From U'_{k+3} + U''_{k+3} = 0 [where U'_{k+3} and U''_{k+3} are last non-zero digits] we have: U'_{k+3} = n – v > 0.
9) From U' + U'' = 0 we have: U'' < 0 [but U''_{k+3} > 0]; hence:
10) The positive number (–U'') has negative last digit (–v); hence [cf. Lemma]:
11) U' =/ (–U''), hence:
12) U = U' + U" = a^n + b^n – c^n =/ 0.
"What about my counter example in base 3?
For k=1, n^(k+1)=100 and n^(k+2)=1000. Let a=11, b=12 and c = 10 such that a+b-c = 20. K = 1 then 1^3+2^3-0^3 = U' = 100 and a^n+b^n-c^n = 20000 = U''+U' = U''+100. However, 20000 is not equal to 100 + 100*(1+1-1)+1000*P. Thus this part of Victor's proof is clearly incorrect." Here U'_{k+3} = U'_4 = 0 but U''_4 = 2. Also, U'_(k+2) = 100 =/ 0."
In your example do not fail a condition: U''+U' = 0.
"P.S. Have you actually computed U' and U'' in your example where a=b=422 and c=444 in base 5? If so, what are they?"
422^5 + 422^5 – 444^5 = minimum = d [if a=b=422 and c=444; here 22 + 22 = 44, u_3 = 4 + 4 – 4 = 4, or "9"].
Really, 421^5 + 422^5 – 443^5 > d, 423^5 + 421^5 – 444^5 > d, etc.
And 444^5 + 400^5 – 444^5 = maximum.
V.S.