A228444 Primes expressible as concatenation of phi(sigma(n)) and sigma(phi(n)).

11, 23, 43, 67, 127, 631, 839, 3631, 1831, 4831, 3691, 6491, 108127, 80363, 108127, 336403, 132961, 288403, 144961, 504403, 384403, 192961, 384403, 384403, 384403, 432511, 864511, 384961, 432961, 640847, 864511, 640847, 720961, 5441651, 7681651, 8641651, 8641651

Offset

1,1

Links

K. D. Bajpai, Table of n, a(n) for n = 1..1020

Example

a(6)= 631: phi(sigma(17))= 6. sigma(phi(17))= 31. After concatenating gives 631 which is prime.
a(9)= 1831: phi(sigma(34))= 18. sigma(phi(34))= 31. After concatenating gives 1831 which is prime.

Maple

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);

Mathematica

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 *)

Crossrefs

Cf. A062401 (phi(sigma(n))).

Cf. A062402 (sigma(phi(n))).

Sequence in context: A357364 A199848 A352494 * A191235 A146451 A195043

Adjacent sequences: A228441 A228442 A228443 * A228445 A228446 A228447

Keyword

nonn,base,less

Author

K. D. Bajpai, Nov 03 2013

Status

approved