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%I #17 Apr 18 2014 11:35:27
%S 11,23,43,67,127,631,839,3631,1831,4831,3691,6491,108127,80363,108127,
%T 336403,132961,288403,144961,504403,384403,192961,384403,384403,
%U 384403,432511,864511,384961,432961,640847,864511,640847,720961,5441651,7681651,8641651,8641651
%N Primes expressible as concatenation of phi(sigma(n)) and sigma(phi(n)).
%H K. D. Bajpai, <a href="/A228444/b228444.txt">Table of n, a(n) for n = 1..1020</a>
%e a(6)= 631: phi(sigma(17))= 6. sigma(phi(17))= 31. After concatenating gives 631 which is prime.
%e a(9)= 1831: phi(sigma(34))= 18. sigma(phi(34))= 31. After concatenating gives 1831 which is prime.
%p with(numtheory): with(StringTools): KD:= proc() local a; a:= parse(cat(phi(sigma(n)), sigma(phi(n)))); if isprime(a) then return (a) : fi; end: seq(KD(), n=1..10000);
%t ccpr[n_]:=Module[{a=EulerPhi[DivisorSigma[1,n]],b= DivisorSigma[1, EulerPhi[n]]}, FromDigits[Join[IntegerDigits[a],IntegerDigits[b]]]]; Select[Array[ccpr,3000],PrimeQ] (* _Harvey P. Dale_, Apr 18 2014 *)
%Y Cf. A062401 (phi(sigma(n))).
%Y Cf. A062402 (sigma(phi(n))).
%K nonn,base,less
%O 1,1
%A _K. D. Bajpai_, Nov 03 2013
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