

A100827


Highly cototient numbers: records for a(n) in A063741.


5



2, 4, 8, 23, 35, 47, 59, 63, 83, 89, 113, 119, 167, 209, 269, 299, 329, 389, 419, 509, 629, 659, 779, 839, 1049, 1169, 1259, 1469, 1649, 1679, 1889, 2099, 2309, 2729, 3149, 3359, 3569, 3989, 4199, 4289, 4409, 4619, 5249, 5459, 5879, 6089, 6509, 6719, 6929
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OFFSET

1,1


COMMENTS

Each number k on this list has more solutions to the equation x  phi(x) = k (where phi is Euler's totient function, A000010) than any preceding k except 1.
This sequence is a subset of A063741. As noted in that sequence, there are infinitely many solutions to x  phi(x) = 1. Unlike A097942, the highly totient numbers, this sequence has many odd numbers besides 1.
With the expection of 2, 4, 8, all of the known terms are congruent to 1 mod a primorial (A002110). The specific primorial satisfying this congruence would result in a sequence similar to A080404 a(n)=A007947[A055932(n)].  Wilfredo Lopez (chakotay147138274(AT)yahoo.com), Dec 28 2006
Because most of the solutions to x  phi(x) = k are semiprimes p*q with p+q=k+1, it appears that this sequence eventually has terms that are one less than the Goldbachrelated sequence A082917. In fact, terms a(108) to a(176) are A082917(n)1 for n=106..174. [From T. D. Noe, Mar 16 2010]


LINKS

T. D. Noe, Table of n, a(n) for n=1..176
Wikipedia, Highly cototient number


EXAMPLE

a(3) = 8 since x  phi(x) = 8 has three solutions, {12, 14, 16}, one more than a(2) = 4 which has two solutions, {6, 8}.


MATHEMATICA

searchMax = 4000; coPhiAnsYldList = Table[0, {searchMax}]; Do[coPhiAns = m  EulerPhi[m]; If[coPhiAns <= searchMax, coPhiAnsYldList[[coPhiAns]]++ ], {m, 1, searchMax^2}]; highlyCototientList = {2}; currHigh = 2; Do[If[coPhiAnsYldList[[n]] > coPhiAnsYldList[[currHigh]], highlyCototientList = {highlyCototientList, n}; currHigh = n], {n, 2, searchMax}]; Flatten[highlyCototientList]


CROSSREFS

Cf. A063741, A097942, A007374, A101373.
Sequence in context: A002075 A122623 A124014 * A034906 A018323 A151380
Adjacent sequences: A100824 A100825 A100826 * A100828 A100829 A100830


KEYWORD

nonn


AUTHOR

Alonso del Arte, Jan 06 2005


EXTENSIONS

More terms from Robert G. Wilson v, Jan 08 2005


STATUS

approved



