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 A271550 Numbers n such that n is a squarefree semiprime (i.e., omega(n) = 2 = Omega(n)) and phi(n) + 1 is a prime. 1

%I

%S 6,10,14,21,22,26,34,38,46,55,57,58,62,74,77,82,86,91,93,94,95,106,

%T 111,115,118,119,122,133,134,142,145,146,158,166,178,194,202,206,209,

%U 214,217,218,221,226,237,254,262,274,278,287,291,295,298,302,305,314,319,326,329

%N Numbers n such that n is a squarefree semiprime (i.e., omega(n) = 2 = Omega(n)) and phi(n) + 1 is a prime.

%C Equals (A001358 intersection A039698) - A001248.

%H Harvey P. Dale, <a href="/A271550/b271550.txt">Table of n, a(n) for n = 1..1000</a>

%e 21 is in the sequence, because 21 = 3*7 is a semiprime with omega(21) = 2 and phi(21) + 1 = 2*6 + 1 = 13 is a prime.

%e 55 is in the sequence, because 55 = 5*11 is a semiprime with omega(55) = 2 and phi(55) + 1 = 4*10 + 1 = 41 is a prime.

%t Select[Range[400],SquareFreeQ[#]&&PrimeOmega[#]==2&&PrimeQ[EulerPhi[ #]+ 1]&] (* _Harvey P. Dale_, Aug 08 2020 *)

%o (PARI) is(n)=my(f=factor(n)); f[, 2]==[1, 1]~ && isprime((f[1, 1]-1)*(f[2, 1]-1)+1) \\ _Charles R Greathouse IV_, Jul 21 2016

%Y Cf. A001358, A039698, A001248.

%K nonn

%O 1,1

%A _Debapriyay Mukhopadhyay_, Jul 13 2016

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Last modified June 17 04:36 EDT 2021. Contains 345080 sequences. (Running on oeis4.)