%I A077426
%S A077426 5,13,17,29,37,41,53,61,65,73,85,89,97,101,109,113,125,137,145,149,157,
%T A077426 173,181,185,193,197,229,233,241,257,265,269,277,281,293,313,317,325,
%U A077426 337,349,353,365,373,389,397
%N A077426 Numbers n such that continued fraction expansion of (sqrt(n)+1)/2 has 
               odd (primitive) period length.
%C A077426 Numbers n such that Pell equation x^2 - n*y^2 = -4 has infinitely many 
               (integer) solutions. See A078356 and A078357.
%D A077426 O. Perron, "Die Lehre von den Kettenbruechen, Bd.I", Teubner, 1954, 1957 
               (Sec. 30, table p. 108).
%p A077426 isOddPrim := proc(n::integer) local cf; cf := numtheory[cfrac]((sqrt(n)+1)/
               2,'periodic','quotients') ; if nops(op(2,cf)) mod 2 =1 then RETURN(true) 
               ; else RETURN(false) ; fi ; end: notA077426 := proc(n::integer) if 
               issqr(n) then RETURN(true) ; elif not isOddPrim(n) then RETURN(true) 
               ; else RETURN(false) ; fi ; end: A077426 := proc(n::integer) local 
               resul,i ; resul := 5 ; i := 1 ; while i < n do resul := resul+4 ; 
               while notA077426(resul) do resul := resul+4 ; od ; i:= i+1 ; od ; 
               RETURN(resul) ; end: for n from 1 to 61 do print(A077426(n)) ; od 
               : - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Apr 25 2006
%Y A077426 A subsequence of A077425.
%Y A077426 Cf. A077427.
%Y A077426 Sequence in context: A119321 A078900 A113482 this_sequence A002144 A111055 
               A145016
%Y A077426 Adjacent sequences: A077423 A077424 A077425 this_sequence A077427 A077428 
               A077429
%K A077426 nonn,easy
%O A077426 1,1
%A A077426 Wolfdieter Lang (wolfdieter.lang(AT)physik.uni-karlsruhe.de), Nov 29 
               2002