

A050931


Numbers having a prime factor congruent to 1 mod 6.


9



7, 13, 14, 19, 21, 26, 28, 31, 35, 37, 38, 39, 42, 43, 49, 52, 56, 57, 61, 62, 63, 65, 67, 70, 73, 74, 76, 77, 78, 79, 84, 86, 91, 93, 95, 97, 98, 103, 104, 105, 109, 111, 112, 114, 117, 119, 122, 124, 126, 127, 129, 130, 133, 134, 139, 140, 143, 146, 147, 148, 151
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OFFSET

1,1


COMMENTS

Original definition: Solutions c of cot(2*Pi/3)*((a+b+c)*(a+b+c)*(a+bc)*(a+bc))^(1/2)=a^2+b^2c^2, c>a,b integers.
Note cot(2*Pi/3) = 1/sqrt(3).
Also the cvalues for solutions to c^2 = a^2 + ab + b^2 in positive integers. Also the numbers which occur as the longest side of some triangle with integer sides and a 120degree angle.  Paul Boddington, Nov 05 2007
The sequence can also be defined as the numbers w which are Heronian means of two distinct positive integers u and v, i.e., w = [u+sqrt(uv)+v]/3. E.g., 28 is the Heronian mean of 4 and 64 (and also of 12 and 48).  Pahikkala Jussi, Feb 16 2008
From JeanChristophe Hervé, Nov 24 2013: (Start)
This sequence is the analog of hypotenuse numbers A009003 for triangles with integer sides and a 120degree angle. There are two integers a and b > 0 such that a(n)^2 = a^2 + ab + b^2, and a, b and a(n) are the sides of the triangle: a(n) is the sequence of lengths of the longest side of these triangles. A004611 is the same for primitive triangles.
a and b cannot be equal because sqrt(3) is not rational. Then the values a(n) are such that a(n)^2 is in A024606. It follows that a(n) is the sequence of multiples of primes of form 6k+1 A002476.
The sequence is closed under multiplication. The primitive elements are those with exactly one prime divisor of the form 6k+1 with multiplicity one, which are also those for which there exists a unique 120degree integer triangle with its longest side equals to a(n).
(End)


LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Bojan Mohar, Hermitian adjacency spectrum and switching equivalence of mixed graphs, arXiv preprint arXiv:1505.03373 [math.CO], 2015.
Planet Math, Truncated cone
Eric Weisstein's World of Mathematics, Triangle  see especially (19)
Index entries for sequences related to A2 = hexagonal = triangular lattice


FORMULA

A005088(a(n)) > 0. Terms are obtained by the products A230780(k)*A004611(p) for k, p > 0, ordered by increasing values.  JeanChristophe Hervé, Nov 24 2013
cot(2*Pi/3) = 1/sqrt(3) = 0.57735... =  A020760.  M. F. Hasler, Aug 18 2016


MATHEMATICA

Select[Range[2, 200], MemberQ[Union[Mod[#, 6]&/@FactorInteger[#][[All, 1]]], 1]&] (* Harvey P. Dale, Aug 24 2019 *)


PROG

(Haskell)
a050931 n = a050931_list !! (n1)
a050931_list = filter (any (== 1) . map (flip mod 6) . a027748_row) [1..]
 Reinhard Zumkeller, Apr 09 2014
(PARI) is_A050931(n)=n>6&&Set(factor(n)[, 1]%6)[1]==1 \\ M. F. Hasler, Mar 04 2018


CROSSREFS

Cf. A002476, A004611, A024606, A230780 (complement), A009003.
Cf. A027748.
Sequence in context: A332480 A233593 A013651 * A072864 A232437 A275201
Adjacent sequences: A050928 A050929 A050930 * A050932 A050933 A050934


KEYWORD

easy,nonn


AUTHOR

Klaus Strassburger (strass(AT)ddfi.uniduesseldorf.de), Dec 30 1999


EXTENSIONS

Simpler definition from M. F. Hasler, Mar 04 2018


STATUS

approved



