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A291043
Numbers n such that psi(n) = psi(n+1), where psi(n) is Dedekind psi function (A001615).
2
4, 8, 14, 15, 32, 44, 45, 62, 63, 75, 135, 188, 195, 567, 608, 663, 704, 825, 956, 957, 1023, 1034, 1275, 1334, 1484, 1634, 1845, 1935, 2223, 2534, 2685, 2751, 2871, 3195, 3404, 3843, 3915, 4994, 7004, 7315, 7544, 8024, 8055, 9207, 10695, 11205, 11984, 12032
OFFSET
1,1
COMMENTS
The only solutions to psi(n) = psi(n+1) = psi(n+2) below 10^8 are 14, 44, 62, 956.
In this sequence, smallest terms k such that k and k + 1 are both product of m + 1 distinct primes are 14, 1334, 84134, 3571905, 424152105 for 1 <= m <= 5. - Altug Alkan, Aug 17 2017
LINKS
EXAMPLE
4 is in the sequence since psi(4) = psi(5) = 6.
MATHEMATICA
psi[n_] := If[n < 1, 0, n Sum[MoebiusMu[d]^2 / d, {d, Divisors @ n}]];
Select[Range[12000], psi[#] == psi[# + 1] &]
SequencePosition[Table[If[n<1, 0, n Sum[MoebiusMu[d]^2/d, {d, Divisors[n]}]], {n, 13000}], {x_, x_}][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Feb 22 2018 *)
PROG
(PARI) a001615(n) = n*sumdivmult(n, d, issquarefree(d)/d);
isok(n) = a001615(n)==a001615(n+1) \\ Altug Alkan, Aug 17 2017, after Charles R Greathouse IV at A001615
CROSSREFS
Sequence in context: A312242 A312243 A166054 * A312244 A182841 A312245
KEYWORD
nonn
AUTHOR
Amiram Eldar, Aug 16 2017
STATUS
approved