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

1,1

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.

Links

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

Example

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

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]

Crossrefs

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

Sequence in context: A230214 A284876 A216268 * A247679 A344013 A225930

Adjacent sequences: A098215 A098216 A098217 * A098219 A098220 A098221

Keyword

nonn,base

Author

Labos Elemer, Oct 22 2004

Status

approved