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A217006 Numbers k such that phi(k-6) = phi(k) = phi(k+6).

%I A217006 #24 Jul 08 2025 16:31:54
%S A217006 78,84,222,228,438,738,948,1158,3252,5058,5268,6918,7212,7362,8082,
%T A217006 8292,9732,12108,13842,14562,15348,15558,15858,17148,17502,18372,
%U A217006 22482,23202,26148,26652,30468,33492,34212,38028,39828,39972,41988,44508,49332,54738,55092
%N A217006 Numbers k such that phi(k-6) = phi(k) = phi(k+6).
%H A217006 T. D. Noe, <a href="/A217006/b217006.txt">Table of n, a(n) for n = 1..1000</a>
%H A217006 F. Firoozbakht, <a href="http://www.primepuzzles.net/puzzles/puzz_466.htm">Puzzle 466. phi(n-1)=phi(n)=phi(n+1)</a>, in C. Rivera's Primepuzzles.
%t A217006 Select[Range[7, 60000], EulerPhi[# - 6] == EulerPhi[#] == EulerPhi[# + 6] &] (* _T. D. Noe_, Sep 24 2012 *)
%t A217006 Flatten[Position[Partition[EulerPhi[Range[60000]],13,1],_?(#[[1]] == #[[7]] == #[[13]]&),{1},Heads->False]]+6 (* _Harvey P. Dale_, Jun 22 2015 *)
%o A217006 (PARI) ffd(lim,d=6) = {for (i=1+d, lim-d, m = eulerphi(i);if ((m == eulerphi(i-d)) && (m == eulerphi(i+d)),print1(i,", ");););}
%o A217006 (PARI) A217006_print(Nmax,d=6)={my(m=2*d+1,o=vector(m,i,eulerphi(if(i>1,i-1,m)))); for(i=m,Nmax+2*d,(o[(i-d)%m+1]==o[i%m+1]=eulerphi(i))||next; o[(i-2*d)%m+1]==o[i%m+1] & print1(i-d", "))} \\ - _M. F. Hasler_, Sep 23 2012
%Y A217006 Cf. A000010, A179188.
%K A217006 nonn
%O A217006 1,1
%A A217006 _Michel Marcus_, Sep 23 2012