A327880 Numbers k such that phi(k) < phi(k+1) < phi(k+2) < phi(k+3) where phi is the Euler totient function (A000010).

1484, 2534, 3002, 3674, 3926, 4454, 4484, 4784, 4844, 5264, 5312, 5984, 6104, 7994, 8294, 8414, 8774, 8834, 9074, 9164, 9944, 10004, 10724, 11024, 11684, 11894, 12254, 13034, 13064, 13166, 13454, 13754, 14234, 15344, 15554, 16184, 16214, 16814, 17384, 17534

Offset: 1

Author

Amiram Eldar, Sep 28 2019

Keywords

• nonn

Examples

			1484 is in the sequence since phi(1484) = 624, phi(1485) = 720, phi(1486) = 742, and phi(1487) = 1486, and 624 < 720 < 742 < 1486.
		

References

• Jean-Marie De Koninck, Those Fascinating Numbers, American Mathematical Society, 2009, p. 34, entry 105 and p. 130, entry 1484.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A078776, A161962.

Programs

Mathematica

aQ[n_] := AllTrue[Differences @ EulerPhi[n + Range[0, 3]], # > 0 &]; Select[Range[18000], aQ]

PARI

ok(k)={for(i=0, 2, if(eulerphi(k+i) >= eulerphi(k+i+1), return(0))); 1}
{ select(ok, [1..20000]) } \\ Andrew Howroyd, Sep 28 2019