A215029 - OEIS

%I #19 Dec 01 2018 09:28:13

%S 0,1,2,1,-1,0,0,1,1,2,-1,0,0,1,1,2,-1,0,0,1,1,2,-1,0,0,1,0,1,0,1,1,2,

%T 0,1,0,1,0,1,-1,0,1,2,0,1,-1,0,1,2,0,1,0,1,0,1,-1,0,1,2,-1,0,0,1,1,2,

%U -1,0,1,2,0,1,0,1,-1,0,0,1,0,1,0,1,1,2,-1,0,0,1,0,1,0,1,1,2,-1,0,1,2,-1,0,0,1,0,1,1,2,0,1,0,1,-1,0,0,1,0,1,1

%N To get a(n), start with m=n, let p = largest prime < m, set m = m-p if m>0, or m=m+p if m <= 0; repeat until p=2 has been processed; set a(n) = m.

%H Antti Karttunen, <a href="/A215029/b215029.txt">Table of n, a(n) for n = 0..20000</a>

%H Antti Karttunen, <a href="/A215029/a215029.txt">Data supplement: n, a(n) computed for n = 0..100003</a>

%H StackExchange, <a href="http://math.stackexchange.com/questions/176394/a-prime-number-pattern">A prime number pattern</a>, Jul 29 2012.

%p f:=proc(n) local m,a,i,p;

%p if n <= 2 then RETURN(n); fi;

%p m:=n; a:=n;

%p for i from 1 to n do

%p p:=prevprime(m);

%p if a>0 then a:=a-p else a:=a+p; fi;

%p m:=p;

%p if m <= 2 then RETURN(a); fi;

%p od;

%p # should never reach here

%p print("ERROR");

%p end;

%p [seq(f(i),i=0..120)];

%o (PARI) A215029(n) = if(n<=2,n,my(mp=precprime(n-1),d=n); while(mp>0, if(d>0, d -= mp, d += mp); mp = precprime(mp-1)); (d)); \\ _Antti Karttunen_, Nov 28 2018

%Y Cf. A215030-A215035, A214912.

%K sign

%O 0,3

%A _N. J. A. Sloane_, Aug 05 2012