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A062325 Numbers k for which phi(prime(k)) is a square. 7

%I

%S 1,3,7,12,26,45,55,79,106,123,211,252,422,446,595,723,907,1019,1101,

%T 1448,1595,1687,1797,1849,1949,2058,2393,2516,2703,2819,3146,3339,

%U 3477,3626,4353,4437,4590,5153,5398,5653,5836,6276,6543,6736,6911,7207,7695

%N Numbers k for which phi(prime(k)) is a square.

%C Also A002496 indexed by A000040.

%H Harry J. Smith, <a href="/A062325/b062325.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A000720(A002496(n)).

%F A000040(a(n)) = A002496(n).

%e 79 is in the sequence because the 79th prime is 401 and phi(401) is 400 = 20^2.

%e 595 is in the sequence because the 595th prime is 4357 and phi(4357) is 4356 = 66^2.

%t Flatten[Position[Table[IntegerQ[Sqrt[Prime[w]-1]], {w, 1, 25000}], True]]

%t Flatten[Position[EulerPhi[Prime[Range[8000]]],_?(IntegerQ[Sqrt[#]]&)]] (* _Harvey P. Dale_, Apr 23 2014 *)

%o (PARI) for(n=1,1600, if(issquare(eulerphi(prime(n))),print(n)))

%o (PARI) { default(primelimit, 2*10^8); n=m=0; forprime (p=2, 2*10^8, m++; if (issquare(eulerphi(p)), write("b062325.txt", n++, " ", m); if (n==1000, break)) ) } \\ _Harry J. Smith_, Aug 05 2009

%Y Cf. A000040, A000720, A001912, A002496, A005574, A062325, A090693.

%K easy,nonn

%O 1,2

%A _Jason Earls_, Jul 05 2001

%E More terms from _Labos Elemer_, Jul 09 2001

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Last modified September 24 19:19 EDT 2018. Contains 315356 sequences. (Running on oeis4.)