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A320391 Numbers k such that phi(k - 2) = phi(k) - 2. 1
5, 7, 8, 13, 14, 16, 19, 20, 22, 31, 43, 46, 61, 64, 73, 94, 103, 109, 118, 139, 151, 166, 181, 193, 199, 214, 229, 241, 256, 271, 283, 313, 334, 349, 358, 421, 433, 454, 463, 523, 526, 571, 601, 619, 643, 661, 694, 718, 766, 811, 823, 829, 859, 883, 934, 958 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A001838(n)+2. - Robert Israel, Oct 30 2018

EXAMPLE

7 is in the sequence because phi(5) = 4 = phi(7) - 2.

8 is in the sequence because phi(6) = 2 = phi(8) - 2.

9 is not in the sequence because phi(7) = 6 but phi(9) - 2 = 4 instead.

MAPLE

with(numtheory): select(k->phi(k-2)=phi(k)-2, [$1..960]); # Muniru A Asiru, Oct 28 2018

MATHEMATICA

Select[Range@1000, EulerPhi@(# - 2) == EulerPhi[#] - 2 &]

PROG

(MAGMA) [n: n in [3..1000] | EulerPhi(n-2) eq EulerPhi(n)-2];

(PARI) isok(n) = eulerphi(n-2) == eulerphi(n)-2; \\ Michel Marcus, Oct 14 2018

(GAP) Filtered([1..960], k->Phi(k-2)=Phi(k)-2); # Muniru A Asiru, Oct 28 2018

CROSSREFS

Cf. A001838.  Contains A006512 and terms > 10 in A194593.

Sequence in context: A257772 A314374 A066001 * A047477 A216555 A288151

Adjacent sequences:  A320388 A320389 A320390 * A320392 A320393 A320394

KEYWORD

nonn

AUTHOR

Vincenzo Librandi, Oct 13 2018

STATUS

approved

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Last modified February 22 18:24 EST 2020. Contains 332148 sequences. (Running on oeis4.)