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A270667 Nonprime and squarefree Löschian numbers (A003136). 1
1, 21, 39, 57, 91, 93, 111, 129, 133, 183, 201, 217, 219, 237, 247, 259, 273, 291, 301, 309, 327, 381, 399, 403, 417, 427, 453, 469, 471, 481, 489, 511, 543, 553, 559, 579, 589, 597, 633, 651, 669, 679, 687, 703, 721, 723, 741, 763, 777, 793, 813, 817, 831, 849, 871, 889, 903, 921, 939, 949, 973, 993 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
5187 is the first term that has 4 prime divisors.
LINKS
EXAMPLE
21 is a term because 21 = 3*7 = 4^2 + 4*1 + 1^2.
MATHEMATICA
Select[Range[10^3], And[SquareFreeQ@ #, ! PrimeQ@ #, Resolve[Exists[{x, y}, Reduce[# == x^2 + x y + y^2, {x, y}, Integers]]]] &] (* Michael De Vlieger, Mar 21 2016, after Jean-François Alcover at A003136 *)
PROG
(PARI) x='x+O('x^1000); p=eta(x)^3/eta(x^3); for(n=0, 999, if(polcoeff(p, n) != 0 && issquarefree(n) && !isprime(n), print1(n, ", ")));
CROSSREFS
Sequence in context: A338552 A339002 A339004 * A020220 A251122 A084856
KEYWORD
nonn
AUTHOR
Altug Alkan, Mar 21 2016
STATUS
approved

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Last modified February 21 02:30 EST 2024. Contains 370219 sequences. (Running on oeis4.)