cp's OEIS Frontend

A265775 Denominators of upper primes-only best approximates (POBAs) to sqrt(2); see Comments.

%I A265775 #13 Apr 05 2019 17:45:33
%S A265775 2,13,37,43,139,149,313,347,593,743,883,1009,2617,12269,15731,37879,
%T A265775 43789,90533
%N A265775 Denominators of upper primes-only best approximates (POBAs) to sqrt(2); see Comments.
%C A265775 Suppose that x > 0. A fraction p/q of primes is an upper primes-only best approximate, and we write "p/q is in U(x)", if p'/q < x < p/q < u/v for all primes u and v such that v < q, where p' is greatest prime < p in case p >= 3.
%C A265775 Let q(1) = 2 and let p(1) be the least prime >= x. The sequence U(x) follows inductively: for n >= 1, let q(n) is the least prime q such that x < p/q < p(n)/q(n) for some prime p. Let q(n+1) = q and let p(n+1) be the least prime p such that x < p/q < p(n)/q(n).
%C A265775 For a guide to POBAs, lower POBAs, and upper POBAs, see A265759.
%e A265775 The upper POBAs to sqrt(2) start with 3/2, 19/13, 53/37, 61/43, 197/139, 211/149. For example, if p and q are primes and q > 139, and p/q > sqrt(2), then 197/139 is closer to sqrt(2) than p/q is.
%t A265775 x = Sqrt[2]; z = 200; p[k_] := p[k] = Prime[k];
%t A265775 t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265775 d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
%t A265775 t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265775 d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
%t A265775 v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
%t A265775 b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
%t A265775 y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265776/A265777 *)
%t A265775 Numerator[tL]   (* A265772 *)
%t A265775 Denominator[tL] (* A265773 *)
%t A265775 Numerator[tU]   (* A265774 *)
%t A265775 Denominator[tU] (* A265775 *)
%t A265775 Numerator[y]    (* A265776 *)
%t A265775 Denominator[y]  (* A265777 *)
%Y A265775 Cf. A000040, A265759, A265772, A265773, A265774, A265776, A265777.
%K A265775 nonn,frac,more
%O A265775 1,1
%A A265775 _Clark Kimberling_, Dec 20 2015
%E A265775 a(14)-a(18) from _Robert Price_, Apr 05 2019