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%I #14 Apr 28 2018 13:12:34
%S 5,11,71,2591,208391,16692551,48502931,92012201,249206231,419445251,
%T 496978301,1329067391,1837151681,2277479051,2647600061,4733566391,
%U 6435087011,10327948751,14089345691,14923624031,22415286251,27508270301,39662281331,59013882071,70353395351
%N Lesser of twin primes p for which phi(p-1) = phi(p+1), where phi(n) is the Euler totient function (A000010).
%C Intersection of A001359 and A067890 (or A066812).
%C The terms below 10^8 were taken from the paper by Garcia et al.
%H Stephan Ramon Garcia, Elvis Kahoro and Florian Luca, <a href="https://doi.org/10.1080/10586458.2017.1360809">Primitive root bias for twin primes</a>, Experimental Mathematics (2017), pp. 1-10, <a href="http://pages.pomona.edu/~sg064747/PAPERS/PRBTP.pdf">alternative link</a>, preprint, <a href="https://arxiv.org/abs/1705.02485">arXiv:1705.02485</a> [math.NT], 2017.
%e p = 5 is the lesser of the twin primes (5, 7), and phi(5-1) = phi(5+1) = 2.
%t seq={}; Do[p = Prime[i]; If[PrimeQ[p+2] && EulerPhi[p-1] == EulerPhi[p+1], AppendTo[seq, p]], {i, 1, 1000000}]; seq
%o (PARI) isok(p) = isprime(p) && isprime(p+2) && (eulerphi(p-1) == eulerphi(p+1)); \\ _Michel Marcus_, Apr 26 2018
%Y Cf. A000010, A001359, A006512, A008330, A066812, A067890, A067933, A286714, A286715.
%K nonn
%O 1,1
%A _Amiram Eldar_, Apr 26 2018
%E a(12)-a(16) from _Michel Marcus_, Apr 26 2018
%E a(17)-a(25) from _Giovanni Resta_, Apr 26 2018