OFFSET
1,1
COMMENTS
Numbers k for which the maximum of the function log(k)/log(N(x,k-x,k)) occurs only for a single value of x (x < k-x, function N(x,k-x,k) is the squarefree kernel of x*(k-x)*k and gcd(x,k-x,k)=1).
Equivalent description without the use of the logarithmic function: Let R(n,k) = rad(n*k*(n-k)) if n is prime to k and otherwise +oo. Also let L(n) = [R(n,k) for k = 1..n]. Then m is in this list <=> min(L(m)) occurs exactly once in L(m). (All minima are listed in A147298.) - Peter Luschny, Aug 05 2019
EXAMPLE
Using the equivalent description the rows for prime numbers begin:
[2] [2]
[3] [6]
[5] [10, 30]
[7] [42, 70, 42]
[11] [110, 66, 66, 154, 330]
[13] [78, 286, 390, 78, 130, 546]
[17] [34, 510, 714, 442, 510, 1122, 1190, 102]
[19] [114, 646, 114, 570, 1330, 1482, 798, 418, 570]
2, 3, 5 and 17 are on the list because the minimum in their row is unique, 7, 11, 19 do not occur because the minimum is more than once in the row.
MAPLE
rad := n -> mul(k, k in numtheory:-factorset(n)):
g := (n, k) -> `if`(igcd(n, k) = 1, 1, infinity):
L := n -> [seq(g(n, k)*rad(n*k*(n-k)), k=1..n/2)]:
isA172121 := n -> nops([ListTools:-SearchAll(min(L(n)), L(n))]) = 1:
select(isA172121, [$1..87]); # Peter Luschny, Aug 05 2019
PROG
(PARI) rad(n) = factorback(factorint(n)[, 1]); \\ A007947
isok(n) = {my(v = vector(n\2, k, if (gcd([k, n, n-k]) == 1, rad(k*(n-k)*n), oo))); if (#v, #select(x->(x==vecmin(v)), v) == 1); } \\ Michel Marcus, Aug 06 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Artur Jasinski, Jan 26 2010
EXTENSIONS
Offset 1 and name corrected by Michel Marcus, Aug 04 2019
Prepended 2 to the list by Peter Luschny, Aug 06 2019
STATUS
approved