A098218 Nonprime numbers whose cototient is a decimal repunits >1 from A002275.

35, 121, 231, 327, 535, 1111, 2047, 2407, 2911, 3127, 3327, 20767, 45967, 64111, 75847, 81607, 103927, 177367, 202207, 210767, 224295, 234607, 275647, 277807, 290911, 295447, 305887, 308911, 321407, 333327, 453911, 475967, 586127, 1199327

Offset: 1

Author

Labos Elemer, Oct 22 2004

Keywords

• nonn
• base

Comments

It is believed that for every repdigit r>1, inverse(cototient(r)) has a solution, usually more than one. For r=1, primes are the solutions.

Examples

			n=1111 and cototient(1111)=111. By accident, both n and its cototient are decimal repunits.
		

Links

• Donovan Johnson, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A096503, A010785, A002275, A098216, A098217.

Programs

Mathematica

ta={{0}};Do[s=Length[u=Union[IntegerDigits[n-EulerPhi[n]]]]; If[Equal[s, 1]&&!PrimeQ[n]&&Equal[u, {1}], Print[{n, n-EulerPhi[n]}]; ta=Append[ta, n]], {n, 1, 100000}];ta=Delete[ta, 1];ta-EulerPhi[ta]