Thread: January 2017
View Single Post
Old 2017-02-15, 16:32   #3
R. Gerbicz
 
R. Gerbicz's Avatar
 
"Robert Gerbicz"
Oct 2005
Hungary

26138 Posts
Default

My sent solution with code(!):

x=(1+1/80)^80
y=(1+1/80)^81
the first and last ten digits of x: 2701484940, 6337890625
the first and last ten digits of y: 2735253502, 1142578125

For positive integer n value it is known that x=(1+1/n)^n and y=(1+1/n)^(n+1)
gives a rational solution of the equation, because:
x^y=((1+1/n)^n)^((1+1/n)^(n+1))=(1+1/n)^((n+1)^(n+1)/n^n)
y^x=((1+1/n)^(n+1))^((1+1/n)^n)=(1+1/n)^((n+1)^(n+1)/n^n)
hence x^y=y^x. If we want x, where we can see the last eight (or some) digits
then n=2^a*5^b for some non-negative a,b integers.
Restricting the search for a,b<10 we have found 3 solutions,
the simple PARI-GP solution, using only integer arithmetic:

for(a=0,9,for(b=0,9,n=2^a*5^b;e=max(a,b);M=10^8;\
r=Mod(n+1,M)^n*Mod(2,M)^(n*(e-a))*Mod(5,M)^(n*(e-b));\
r=lift(r);test=1;v=vector(10,i,0);for(i=1,8,\
res=1+(r%10);r\=10;v[res]+=1;if(v[res]>1,test=0));\
if(test,X=[a,b,n];print([a,b,n]))))

what has found n=80;3125;1953125 as a solution,
this gives for example that x=(1+1/80)^80 and y=(1+1/80)^81 a solution.

A "cut" function used to get the first and last
L digits of a positive rational number:

cut(r,L)={tmp=r;d=10^L;while(type(tmp)!="t_INT",tmp*=10);\
t=tmp;while(tmp>=d,tmp\=10);return([tmp,t%d])}

In the second email:

Just for small correction obvioulsy
x starts as 2.701484940
and y with 2.735253502
if you needed also the first (ten) digits with a decimal point.

ps. use cut() only if r>0 has a finite representation in base 10.
R. Gerbicz is offline   Reply With Quote