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 A057920 Numbers k such that phi(k+1) divides phi(k), where phi is A000010. 2
 1, 3, 5, 13, 15, 35, 37, 61, 73, 104, 119, 157, 164, 193, 194, 255, 277, 313, 397, 421, 455, 457, 495, 527, 541, 545, 584, 613, 629, 661, 665, 673, 733, 757, 877, 975, 997, 1085, 1093, 1153, 1201, 1213, 1237, 1295, 1321, 1381, 1453, 1469, 1621, 1657, 1753 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The intersection of this sequence and A057919 is A001274. - Michel Marcus, Sep 14 2015 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE 13 is included because phi(14) = 6 divides phi(13) = 12. MAPLE for n from 1 to 2000 do     if modp(numtheory[phi](n), numtheory[phi](n+1)) =0 then         printf("%d, ", n) ;     end if; end do: # R. J. Mathar, Sep 14 2015 MATHEMATICA Select[Range[1800], Divisible[EulerPhi[#], EulerPhi[# + 1]] &] (* Amiram Eldar, Jul 13 2019 *) PROG (PARI) lista(nn) = for (n=1, nn, if (eulerphi(n) % eulerphi(n+1) == 0, print1(n, ", "))); \\ Michel Marcus, Sep 14 2015 CROSSREFS Cf. A001274, A057919. Sequence in context: A018753 A073217 A063484 * A018329 A090545 A045411 Adjacent sequences:  A057917 A057918 A057919 * A057921 A057922 A057923 KEYWORD nonn AUTHOR Leroy Quet, Nov 11 2000 STATUS approved

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Last modified December 9 08:41 EST 2021. Contains 349627 sequences. (Running on oeis4.)