[seqfan] Re: some gp/pari code in OEIS returns rationals, not integers

[Richard Mathar]

gg> From seqfan-bounces at list.seqfan.eu Wed Mar 11 15:54:31 2009
gg> Date: Wed, 11 Mar 2009 16:45:15 +0200
gg> From: Georgi Guninski <guninski at guninski.com>
gg> To: seqfan at seqfan.eu
gg> Subject: [seqfan]  some gp/pari code in OEIS returns rationals, not integers
gg> 
gg> some gp/pari code in OEIS returns rationals, not integers
gg> 
gg> A004738 - often

We can repair (re-pari) this by either a PARI code that does not use floating
point arithmetics (preferred), or an alternative one which avoids loops and
might be quicker then if n is large:


A004738(n)={
	local(nred=n-1,i=1) ;
	while(nred-2*i>=0,
		nred -= 2*i;
		i++ ;
	) ;
	return(1+min(nred,2*i-nred)) ;
}

A004738alt(n)={
	local(d= sqrtint(4*n-3)-1,nred) ;
	if(d %2,
		d--
	) ;
	nred=n-1-d/2*(d/2+1) ;
	return(1+min(nred,d+2-nred)) ;
}