A265773 Denominators of lower primes-only best approximates (POBAs) to sqrt(2); see Comments.

2, 5, 29, 691, 773, 971, 1217, 1613, 2207, 2347, 2791, 3467, 3491, 6079, 27073, 45281, 55609

Offset

1,1

Comments

Suppose that x > 0. A fraction p/q of primes is a lower primes-only best approximate, and we write "p/q is in L(x)", if u/v < p/q < x < p'/q for all primes u and v such that v < q, where p' is least prime > p.

Let q(1) be the least prime q such that u/q < x for some prime u, and let p(1) be the greatest such u. The sequence L(x) follows inductively: for n > 1, let q(n) is the least prime q such that p(n)/q(n) < p/q < x for some prime p. Let q(n+1) = q and let p(n+1) be the greatest prime p such that p(n)/q(n) < p/q < x.

For a guide to POBAs, lower POBAs, and upper POBAs, see A265759.

Links

Table of n, a(n) for n=1..17.

Example

The lower POBAs to sqrt(2) start with 2/2, 7/5, 41/29, 977/691, 1093/773, 1373/971. For example, if p and q are primes and q > 691, and p/q < sqrt(2), then 977/691 is closer to sqrt(2) than p/q is.

Mathematica

x = Sqrt[2]; z = 200; p[k_] := p[k] = Prime[k];
t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265776/A265777 *)
Numerator[tL]   (* A265772 *)
Denominator[tL] (* A265773 *)
Numerator[tU]   (* A265774 *)
Denominator[tU] (* A265775 *)
Numerator[y]    (* A265776 *)
Denominator[y]  (* A265777 *)

Crossrefs

Cf. A000040, A265759, A265772, A265774, A265775, A265776, A265777.

Sequence in context: A179823 A064098 A181078 * A098717 A059784 A000283

Adjacent sequences: A265770 A265771 A265772 * A265774 A265775 A265776

Keyword

nonn,frac,more

Author

Clark Kimberling, Dec 20 2015

Extensions

a(15)-a(17) from Robert Price, Apr 05 2019

Status

approved