A339794 a(n) is the least integer k satisfying rad(k)^2 < sigma(k) and whose prime factors set is the same as the prime factors set of A005117(n+1).

4, 9, 25, 18, 49, 80, 121, 169, 112, 135, 289, 361, 441, 352, 529, 416, 841, 360, 961, 891, 1088, 875, 1369, 1216, 1053, 1681, 672, 1849, 1472, 2209, 2601, 2809, 3025, 3249, 1856, 3481, 3721, 1984, 4225, 1584, 4489, 4761, 1960, 5041, 5329, 4736, 5929, 2496, 6241

Offset

1,1

Comments

Equivalently, subsequence of terms of A339744 excluding terms whose prime factor set has already been encountered.

a(n) = A005117(n + 1)^2 when A005117(n + 1) is prime. Proof: if A005117(n + 1) is a prime p then rad(A005117(n + 1))^2 = rad(p)^2 = p^2 and so integers whose prime factors set is the same as the prime factors set of A005117(n + 1) = p are p^m where m >= 1. p^2 > sigma(p^1) = p + 1 but p^2 < sigma(p^2) = p^2 + p + 1. Q.E.D. - David A. Corneth, Dec 19 2020

From Bernard Schott, Jan 19 2021: (Start)

Indeed, a(n) satisfies the double inequality A005117(n+1) < a(n) <= A005117(n+1)^2.

It is also possible that a(n) = A005117(n+1)^2, even when A005117(n+1) is not prime; the smallest such example is for a(13) = 441 = 21^2 = A005117(14)^2. (End)

Links

David A. Corneth, Table of n, a(n) for n = 1..10000 (first 1000 terms from Michel Marcus)

Formula

a(n) <= A005117(n+1)^2. - David A. Corneth, Dec 19 2020

Example

   n  a(n) prime factor set
   1    4  [2]           A000079
   2    9  [3]           A000244
   3   25  [5]           A000351
   4   18  [2, 3]        A033845
   5   49  [7]           A000420
   6   80  [2, 5]        A033846
   7  121  [11]          A001020
   8  169  [13]          A001022
   9  112  [2, 7]        A033847
  10  135  [3, 5]        A033849
  11  289  [17]          A001026
  12  361  [19]          A001029
  13  441  [3, 7]        A033850
  14  352  [2, 11]       A033848
  15  529  [23]          A009967
  16  416  [2, 13]       A288162
  17  841  [29]          A009973
  18  360  [2, 3, 5]     A143207

Prog

(PARI) u(n) = {my(fn=factor(n)[, 1]); for (k = n, n^2, my(fk = factor(k)); if (fk[, 1] == fn, if (factorback(fk[, 1])^2 < sigma(fk), return (k)); ); ); }
lista(nn) = {for (n=2, nn, if (issquarefree(n), print1(u(n), ", "); ); ); }

Crossrefs

Cf. A000203 (sigma), A007947 (rad).

Cf. A005117 (squarefree numbers), A027748, A265668, A339744.

Subsequence: A001248 (squares of primes).

Sequence in context: A131826 A366786 A051961 * A251544 A175119 A093867

Adjacent sequences: A339791 A339792 A339793 * A339795 A339796 A339797

Keyword

nonn

Author

Michel Marcus, Dec 17 2020

Status

approved