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A228444 Primes expressible as concatenation of phi(sigma(n)) and sigma(phi(n)). 1
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 (list; graph; refs; listen; history; text; internal format)
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: A190797 A068842 A199848 * A191235 A146451 A195043

Adjacent sequences:  A228441 A228442 A228443 * A228445 A228446 A228447

KEYWORD

nonn,base,less

AUTHOR

K. D. Bajpai, Nov 03 2013

STATUS

approved

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Last modified December 11 21:15 EST 2019. Contains 329937 sequences. (Running on oeis4.)