PARI/GP code took less than a second: big_fib(N=30,STEP=7)= { local(n=fibonacci(N),d,nd=ceil(log(n)/log(10)),nt=nd\STEP,ynd,str=""); ynd=STEP*nt+1; n\=10^(nd-ynd); forstep(i=1,ynd,STEP, d=n%10; n\=10^STEP; str=concat(Str(d),str); ); str } /* ? big_fib(1500000,20000) %53 = "11612696866...