A328056 - OEIS

%I #22 Sep 08 2022 08:46:24

%S 313,523,733,823,824,943,944,973,1153,1363,1573,1753,1783,1813,1993,

%T 2143,2413,2473,2623,2803,3043,3133,3134,3253,3313,3463,3703,3883,

%U 4093,4123,4303,4387,4388,4513,4723,4873,4874,4933,5113,5143,5353,5443,5444,5563,5564

%N Numbers k such that phi(k) > phi(k+1) > phi(k+2) where phi is the Euler totient function (A000010).

%C Contains all members k of A206581 such that k==103 (mod 210) except 103.- _Robert Israel_, Oct 16 2019

%H Robert Israel, <a href="/A328056/b328056.txt">Table of n, a(n) for n = 1..10000</a>

%e 313 is in the sequence since phi(313) = 312, phi(314) = 156, phi(315) = 144, and 312 > 156 > 144.

%p R:= NULL: count:= 0:

%p q:= false: s:= 0:

%p for i from 1 while count < 100 do

%p   t:= numtheory:-phi(i);

%p   r:= q;

%p   q:= evalb(t<s);

%p   if r and q then count:= count+1; R:= R, i-2 fi;

%p   s:= t;

%p od:

%p R; # _Robert Israel_, Oct 16 2019

%t Flatten[Position[Partition[EulerPhi[Range[5600]], 3, 1], _?(Max[Differences[#]] < 0 &)] // Quiet] (* _Amiram Eldar_, Oct 06 2019 after _Harvey P. Dale_ at A078776 *)

%o (Magma) [k:k in [1..5600]| EulerPhi(k) gt EulerPhi(k+1) and EulerPhi(k+1) gt EulerPhi(k+2)]; // _Marius A. Burtea_, Oct 07 2019

%Y Cf. A000010, A078776, A161962, A161963, A206581, A327880.

%Y Supersequence of A326817.

%K nonn

%O 1,1

%A _Kritsada Moomuang_, Oct 03 2019