OFFSET
1,1
LINKS
Paolo P. Lava, Table of n, a(n) for n = 1..200
FORMULA
a(n) = 4*(prime(n+1) + 1). - Paolo P. Lava, Sep 06 2017
EXAMPLE
16 = 4 + 12 = (4 - phi(4)) * (12 - phi(12)) = 2 * 8 = 16 and also
16 = 8 + 8 = (8 - phi(8)) * (8 - phi(8)) = 4 * 4 = 16;
24 = 4 + 20 = (4 - phi(4)) * (20 - phi(20)) = 2 * 12 = 24.
MAPLE
with(numtheory): P:=proc(q) local a, b, k, n; for n from 1 to q do
for k from 1 to trunc(n/2) do if (k-phi(k))*(n-k-phi(n-k))=n then print(n); break; fi;
od; od; end: P(10^9);
MATHEMATICA
Select[Range@ 1032, Function[n, Length@ Select[Times @@ Map[(# - EulerPhi@ #) &, {#, n - #}] & /@ Range[0, Floor[n/2]], # == n &] > 0]] (* Michael De Vlieger, Jun 01 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Paolo P. Lava, May 31 2016
STATUS
approved