login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A277343 a(1) is 4. a(n) is the least semiprime q (A001358) greater than p = a(n-1), such that p/q is a new minimum. 1
4, 6, 10, 21, 46, 106, 247, 579, 1363, 3214, 7586, 17915, 42311, 99931, 236023, 557455, 1316638, 3109733, 7344803, 17347513, 40972678, 96772393, 228564417, 539840885, 1275037411, 3011480697, 7112745019, 16799424206, 39678162637, 93714913738, 221343037931, 522784885426, 1234753254431 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Inspired by and analogous to A265418.

p/q -> 0.423392190744304142156851442297311481582158896664...

LINKS

Robert G. Wilson v, Table of n, a(n) for n = 1..200

EXAMPLE

4/6 is 0.666... is a new low or minimum;

6/9 is 0.666... is not a new minimum, but;

6/10 is 0.600... is a new minimum;

10/21 is 0.476... is a new minimum;

21/46 is 0.456... is a new minimum;

... 522784885426/1234753254431 is 0.423... is a new minimum; etc.

MAPLE

with(numtheory): P:=proc(q) local a, b, n; a:=4; c:=a; print(a);

for n from a+1 to q do if issqrfree(n) and nops(ifactors(n)[2])=2 then

if a/n<c then c:=a/n; a:=n; print(n); fi; fi; od; end: P(10^6); # Paolo P. Lava, Nov 03 2016

MATHEMATICA

NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[PrimeOmega[sp] != 2, If[sgn < 0, sp--, sp++]]; If[sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]];

p = 4; q = 6; mx = 1; lst = {}; While[q < 10^15, r = p/q; If[r < mx, mx = r; AppendTo[lst, p]; p = q]; q = NextSemiPrime[Floor[q/r]]]; lst (* or *)

f[lst_List] := Block[{p = lst[[-2]], q = lst[[-1]]}, Append[lst, NextSemiPrime[ Floor[q^2/p]]]]; lst = {4, 6}; lst = Nest[f, lst, 30]

CROSSREFS

Cf. A001358, A265418.

Sequence in context: A320124 A185913 A243119 * A077065 A131867 A252656

Adjacent sequences:  A277340 A277341 A277342 * A277344 A277345 A277346

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Oct 09 2016

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 27 17:58 EST 2020. Contains 331296 sequences. (Running on oeis4.)