Sunday, 5 January 2020

number theory - gcd as positive linear combination



Good evening,



I have a question concerning the euclidean algorithm.



One knows that for a1,,anN and kN there exist some λiZ such that :




gcd(a1,,an)=1kni=1λiai



Here is my question: can one find a m0N that for every mm0 there are scalars μiN such that:



gcd(a1,,an)=1mni=1μiai



Unfortunately I have only very rudimentary knowledge about number theory ...



With best regards
Mat



Answer



Let's say that gcd(a1,,an)=d and d=ni=1λiai for some λiZ.
Suppose that siN are sufficiently large such that ri=λi+sia1a2ai1ai+1an>a1d|λi| and ni=1riai=m0d for some m0N.
For all r=0,1,,a1d1 we have (m0+r)d=ni=1(ri+rλi)ai

and ri+rλi>0 for all i.
For ra1d if r=qa1d+s with qN, s{0,1,,a1d1} we have (m0+r)d=(r1+sλ1+q)a1+ni=2(ri+sλi)ai

and r1+sλ1+q>0, ri+sλi>0 for all i2.
Therefore for every mmo,  md=ni=1μiaid=1mni=1μiai for some μiN.


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