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A273801
Numbers n for which n = (x - phi(x)) * (y - phi(y)), where n = x + y and x - phi(x) is the Euler cototient function of x.
3
16, 24, 32, 48, 56, 72, 80, 96, 120, 128, 152, 168, 176, 192, 216, 240, 248, 272, 288, 296, 320, 336, 360, 392, 408, 416, 432, 440, 456, 512, 528, 552, 560, 600, 608, 632, 656, 672, 696, 720, 728, 768, 776, 792, 800, 848, 896, 912, 920, 936, 960, 968, 1008, 1032
OFFSET
1,1
LINKS
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
Sequence in context: A253782 A247065 A082803 * A163284 A100316 A206260
KEYWORD
nonn
AUTHOR
Paolo P. Lava, May 31 2016
STATUS
approved