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A304333 Number of positive integers k such that n - L(k) is a positive squarefree number, where L(k) denotes the k-th Lucas number A000204(k). 9

%I #32 May 15 2018 08:39:27

%S 0,1,1,2,2,3,2,3,3,3,2,3,3,5,2,3,4,5,2,4,4,4,3,5,4,4,2,3,3,5,3,5,5,5,

%T 4,4,5,4,4,6,5,6,3,6,4,5,3,6,5,6,3,5,4,5,3,3,4,6,4,6,4,7,3,6,4,6,2,6,

%U 6,6,4,5,6,4,4,6,7,6,3,7,6,6,4,6,5,7,5,6,7,8

%N Number of positive integers k such that n - L(k) is a positive squarefree number, where L(k) denotes the k-th Lucas number A000204(k).

%C Conjecture: a(n) > 0 for all n > 1.

%C This has been verified for n up to 5*10^9.

%C See also A304331 for a similar conjecture involving Fibonacci numbers.

%C For all n, a(n) <= A130241(n). - _Antti Karttunen_, May 13 2018

%H Antti Karttunen, <a href="/A304333/b304333.txt">Table of n, a(n) for n = 1..65537</a>

%H Zhi-Wei Sun, <a href="http://maths.nju.edu.cn/~zwsun/116f.pdf">Mixed sums of primes and other terms</a>, in: Additive Number Theory (edited by D. Chudnovsky and G. Chudnovsky), pp. 341-353, Springer, New York, 2010.

%H Zhi-Wei Sun, <a href="https://doi.org/10.1007/978-3-319-68032-3_20">Conjectures on representations involving primes</a>, in: M. Nathanson (ed.), Combinatorial and Additive Number Theory II, Springer Proc. in Math. & Stat., Vol. 220, Springer, Cham, 2017, pp. 279-310. (See also <a href="http://arxiv.org/abs/1211.1588">arXiv:1211.1588 [math.NT]</a>, 2012-2017.)

%e a(2) = 1 with 2 - L(1) = 1 squarefree.

%e a(3) = 1 with 3 - L(1) = 2 squarefree.

%e a(67) = 2 with 67 - L(1) = 2*3*11 and 67 - L(7) = 2*19 both squarefree.

%p a := proc(n) local count, lucas, newcas;

%p count := 0; lucas := 1; newcas := 2;

%p while lucas < n do

%p if numtheory:-issqrfree(n - lucas) then count := count + 1 fi;

%p lucas, newcas := lucas + newcas, lucas;

%p od;

%p count end:

%p seq(a(n), n=1..90); # _Peter Luschny_, May 15 2018

%t f[n_]:=f[n]=LucasL[n];

%t tab={};Do[r=0;k=1;Label[bb];If[f[k]>=n,Goto[aa]];If[SquareFreeQ[n-f[k]],r=r+1];k=k+1;Goto[bb];Label[aa];tab=Append[tab,r],{n,1,90}];Print[tab]

%o (PARI) A304333(n) = { my(u1=1,u2=3,old_u1,c=0); if(n<=2,n-1,while(u1<n,c+=issquarefree(n-u1);old_u1=u1;u1=u2;u2=old_u1+u2); c); }; \\ _Antti Karttunen_, May 13 2018

%Y Cf. A000032, A000204, A005117, A102460, A130241, A304034, A304081, A304331.

%K nonn

%O 1,4

%A _Zhi-Wei Sun_, May 11 2018

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Last modified March 28 09:04 EDT 2024. Contains 371240 sequences. (Running on oeis4.)