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Search: id:A098218
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| A098218 |
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Nonprime numbers whose cototient is a decimal repunits >1 from A002275. |
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+0
1
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| 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
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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It is believed that for every repdigit r>1, inverse[cototient[r]] has solution, usually more than one. For r=1, primes are the solutions.
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EXAMPLE
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n=1111 and cototient[1111]=111.By accident, both n and its cototient are decimal repunits.
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MATHEMATICA
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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]
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CROSSREFS
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Cf. A096503, A010785, A002275, A098216, A098217.
Adjacent sequences: A098215 A098216 A098217 this_sequence A098219 A098220 A098221
Sequence in context: A044286 A044667 A132144 this_sequence A120398 A039522 A044367
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KEYWORD
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nonn
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AUTHOR
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Labos E. (labos(AT)ana.sote.hu), Oct 22 2004
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