login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A232437 Numbers whose square is expressible in only one way as x^2+xy+y^2, with x and y > 0. 5

%I

%S 7,13,14,19,21,26,28,31,35,37,38,39,42,43,52,56,57,61,62,63,65,67,70,

%T 73,74,76,77,78,79,84,86,93,95,97,103,104,105,109,111,112,114,117,119,

%U 122,124,126,127,129,130,134,139,140,143,146,148,151,152,154,155,156,157,158,161

%N Numbers whose square is expressible in only one way as x^2+xy+y^2, with x and y > 0.

%C Analog of A084645 for 120-degree angle triangles with integer sides.

%C Numbers with exactly one prime divisor of the form 6k+1 with multiplicity one.

%C Primitive elements of A050931.

%H <a href="/index/Aa#A2">Index entries for sequences related to A2 = hexagonal = triangular lattice</a>

%F Terms are obtained by the products A230780(k)*A002476(p) for k, p > 0, ordered by increasing values.

%e a(1) = 7 as 7^2 = 3^2 + 3*5 + 5^2.

%t selQ[1] = False; selQ[n_] := Module[{f = FactorInteger[n]}, Count[f, {p_, 1} /; Mod[p, 6] == 1] == 1]; Select[Range[200], selQ] (* _Jean-François Alcover_, Nov 25 2013, after comments *)

%Y Cf. A002476, A050931, A230780, A232436 (subsequence).

%Y Cf. A084645, A232437, A248599, A254063, A254064.

%K nonn

%O 1,1

%A _Jean-Christophe Hervé_, Nov 24 2013

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 7 03:00 EST 2019. Contains 329836 sequences. (Running on oeis4.)