A325237 -id:A325237 - cp's OEIS frontend

A325236 Squarefree k such that phi(k)/k - 1/2 is positive and minimal for k with gpf(k) = prime(n).

1, 2, 3, 15, 21, 231, 273, 255, 285, 167739, 56751695, 7599867, 3829070245, 567641679, 510795753, 39169969059, 704463969, 3717740976339, 42917990271, 547701649495, 45484457928390429, 59701280265935165

Offset: 0

Michael De Vlieger, Apr 19 2019

nonn

Let gpf(k) = A006530(k) and let phi(n) = A000010(n) for k in A005117.
There are 2^(n-1) numbers k with gpf(k) = prime(n), since we can only either have p_i^0 or p_i^1 where p_i | k and i <= n. For example, for n = 2, there are only 2 squarefree numbers k with prime(2) = 3 as greatest prime factor. These are 3 = 2^0 * 3^1, and 6 = 2^1 * 3^1. We observe that we can write multiplicities of the primes as A067255(k), and thus for the example derive 3 = "0,1" and 6 = "1,1". Thus for n = 3, we have 5 = "0,0,1", 15 = "0,1,1", 10 = "1,0,1", and 30 = "1,1,1". This establishes the possible values of k with respect to n. We choose the value of k in n for which phi(k)/k - 1/2 is positive and minimal.
We know that prime k (in A000040) have phi(k)/k = A006093(n)/A000040(n) and represent maxima in n. We likewise know primorials k (in A002110) have phi(k)/k = A038110(n)/A060753(n) and represent minima in n. This sequence shows squarefree numbers k with gpf(k) = n such that their value phi(k)/k is closest to but more than 1/2.
Apart from a(1) = 2, all terms are odd. For n > 1 and k even, phi(k)/k - 1/2 is negative.

			First terms of this sequence appear in the chart below between asterisks.
The values of n appear in the header, values of k followed parenthetically by phi(k)/k appear in column n. The x axis plots k according to primepi(gpf(k)), while the y axis plots k according to phi(k)/k:
    0       1          2             3             4
    .       .          .             .             .
-- *1* -----------------------------------------------
  (1/1)     .          .             .             .
    .       .          .             .             .
    .       .          .             .             .
    .       .          .             .             7
    .       .          .             5           (6/7)
    .       .          .           (4/5)           .
    .       .          .             .             .
    .       .          .             .            35
    .       .         *3*            .          (24/35)
    .       .        (2/3)           .             .
    .       .          .             .             .
    .       .          .             .             .
    .       .          .             .           *21*
    .       .          .             .           (4/7)
    .       .          .           *15*            .
    .       .          .          (8/15)           .
    .      *2*         .             .             .
----------(1/2)---------------------------------------
    .       .          .             .             .
    .       .          .             .            105
    .       .          .             .          (16/35)
    .       .          .             .            14
    .       .          .            10           (3/7)
    .       .          .           (2/5)           .
    .       .          .             .             .
    .       .          .             .            70
    .       .          6             .          (12/35)
    .       .        (1/3)           .             .
    .       .          .             .            42
    .       .          .            30           (2/7)
    .       .          .          (4/15)           .
    .       .          .             .            210
    .       .          .             .           (8/35)
...
a(3) = 15 for the following reasons. There are 4 possible values of k with n = 3. These are 5, 15, 10, and 30 with phi(k)/k = 4/5, 8/15, 2/5, and 4/15, respectively. Subtracting 1/2 from each of the latter values, we derive 3/10, 1/30, -1/10, and -7/30 respectively. Since the smallest of these differences is 3/10 pertaining to k = 15, a(3) = 15.

Michael De Vlieger, Plot of A325236(n) among terms in row n of A307540.

Cf. A000010, A000040, A002110, A005117, A006093, A006530, A038110, A060753, A307540, A325237.

With[{e = 15}, Map[MinimalBy[#, If[# < 0, # + 1, #] &[#[[2]] - 1/2] &] &, SplitBy[#, Last]] &@ Array[{#2, EulerPhi[#2]/#2, If[! IntegerQ@ #, 0, #] &[1 + Floor@ Log2@ #1]} & @@ {#, Times @@ MapIndexed[Prime[First@ #2]^#1 &, Reverse@ IntegerDigits[#, 2]]} &, 2^(e + 1), 0]][[All, 1, 1]]

cp's OEIS Frontend

A325236 Squarefree k such that phi(k)/k - 1/2 is positive and minimal for k with gpf(k) = prime(n).

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