[seqfan] Re: Shouldn't A158624 be hard and base?

israel at math.ubc.ca israel at math.ubc.ca
Mon Apr 2 03:20:43 CEST 2012

There may be no known formula, but I wouldn't really call it "hard". My 
Maple program gets the first 1000 terms in about 12 seconds on my computer.

A158624:= proc(N)
local m,n,A;
m[2]:= 3;
for n from 3 to N do
  A:= 5&^m[n-1] mod 10^n;
  if A > 5*10^(n-1) then m[n]:= m[n-1]
  else m[n]:= m[n-1]+2^(n-3)
  end if
end do:
convert(5&^m[N] mod 10^(N),base,10);
end proc; 

Robert Israel
University of British Columbia

On Apr 1 2012, Alonso Del Arte wrote:

>A158624 is the upper limit of the "backward" value of 5^n. I'm thinking
>this should have keyword:base because its value depends on base 10 (in
>quintal it would of course just be 1/5), and keyword:hard because there is
>no known formula.
>P.S. I have chosen this for Sequence of the Day for November
>Seqfan Mailing list - http://list.seqfan.eu/

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