A215029 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.

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, 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, -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

Offset: 0

Author

N. J. A. Sloane, Aug 05 2012

Keywords

• sign

Links

• Antti Karttunen, Table of n, a(n) for n = 0..20000
• Antti Karttunen, Data supplement: n, a(n) computed for n = 0..100003
• StackExchange, A prime number pattern, Jul 29 2012.

Crossrefs

Cf. A215030-A215035, A214912.

Programs

Maple

f:=proc(n) local m,a,i,p;
if n <= 2 then RETURN(n); fi;
m:=n; a:=n;
for i from 1 to n do
p:=prevprime(m);
if a>0 then a:=a-p else a:=a+p; fi;
m:=p;
if m <= 2 then RETURN(a); fi;
od;
# should never reach here
print("ERROR");
end;
[seq(f(i),i=0..120)];

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