%I
%S 23,47,139,529,611,911,1109,1445,1621,2003,2521,2531,4007,4361,4879,
%T 7169,9011,9013,9481,13009,18883,22081,24257,25031,45349,49901,56081,
%U 90011,98941,99101,133705,160031,169181,200003,202289,210181,250031,307289,336961,357101
%N Numbers n such that there are a, b with abs(sigma(a)  sigma(b)) = sigma(n)  n and a U b = n, where U is decimal concatenation.
%H Paolo P. Lava and Giovanni Resta, <a href="/A239563/b239563.txt">Table of n, a(n) for n = 1..207</a>(terms < 3*10^9, first 50 terms from Paolo P. Lava)
%e For n = 9481 we can consider 9481 = 9 U 481 and sigma(9481) = 10000, sigma(481) = 532, sigma(9) = 13 and 532  13 = 519 = 10000  9481.
%p with(numtheory);
%p T:=proc(t) local w,x,y; x:=t; y:=0; while x>0 do x:=trunc(x/10); y:=y+1; od; end:
%p P:=proc(q) local a,b,c,d,i,n;
%p for n from 1 to q do a:=sigma(n); b:=T(n);
%p for i from 1 to b1 do c:=trunc(n/10^i); d:=nc*10^i;
%p if abs(sigma(c)sigma(d))=an then print(n); break; fi;
%p od; od; end: P(10^9);
%Y Cf. A000203, A239562.
%K nonn,base
%O 1,1
%A _Paolo P. Lava_, Mar 21 2014
