%I #9 Jan 12 2013 17:52:04

%S 5,7,11,11,13,17,17,19,23,23,29,29,21,31,37,37,37,41,28,33,47,47,53,

%T 53,53,59,59,44,61,67,67,67,71,57,73,79,79,79,83,83,69,89,74,101,68,

%U 97,97,85,101,103,107,107,109,113,93,131,127,127,131,127,127,127

%N Smallest number k such that sigma(k-2*n)=sigma(k)-2*n.

%C Note all k>=1 are considered, even if k-2n<0. If the search space is k>=2n, variants of A020484 and A060264 appear. - _R. J. Mathar_, Jan 12 2013

%e a(15)=37 because 37 is the minimum number for which sigma(37-2*15)=sigma(7)=8 and sigma(37)-2*15=38-30=8.

%p A206770:=proc(q)

%p local k,n;

%p for n from 1 to q do

%p for k from 1 to q do

%p if sigma(k-2*n)=sigma(k)-2*n then print(k); break; fi;

%p od; od; end:

%p A206770(1000000000);

%p A206770 := proc(n)

%p local k ;

%p for k from 1 do

%p if numtheory[sigma](k-2*n) = numtheory[sigma](k)-2*n then

%p return k;

%p end if;

%p end do:

%p end proc: # _R. J. Mathar_, Jan 12 2013

%Y Cf. A054906

%K nonn

%O 1,1

%A _Paolo P. Lava_, Jan 10 2013