This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A055258 #22 Apr 08 2025 13:56:32
%S A055258 2,8,14,50,56,98,344,350,392,686,2402,2408,2450,2744,4802,16808,16814,
%T A055258 16856,17150,19208,33614,117650,117656,117698,117992,120050,134456,
%U A055258 235298,823544,823550,823592,823886,825944,840350,941192,1647086,5764802,5764808,5764850,5765144,5767202,5781608,5882450,6588344,11529602
%N A055258 Sums of two powers of 7.
%H A055258 T. D. Noe, <a href="/A055258/b055258.txt">Rows n = 0..100 of triangle, flattened</a>
%F A055258 a(n) = 7^(n-trinv(n))+7^trinv(n), where trinv(n) = floor((1+sqrt(1+8*n))/2) = A002262(n) and n-trinv(n) = A003056(n)
%F A055258 Regarded as a triangle T(n, k) = 7^n + 7^k, so as a sequence a(n) = 7^A002262(n) + 7^A003056(n).
%t A055258 t = 7^Range[0, 9]; Select[Union[Flatten[Table[i + j, {i, t}, {j, t}]]], # <= t[[-1]] + 1 &] (* _T. D. Noe_, Oct 09 2011 *)
%t A055258 Total/@Tuples[7^Range[0,10],2]//Union (* _Harvey P. Dale_, Dec 31 2017 *)
%o A055258 (Python)
%o A055258 from math import isqrt
%o A055258 def A055258(n): return 7**(a:=(k:=isqrt(m:=n<<1))+(m>k*(k+1))-1)+7**(n-1-(a*(a+1)>>1)) # _Chai Wah Wu_, Apr 08 2025
%Y A055258 Cf. A052216.
%Y A055258 Equals 2*A073218.
%K A055258 easy,nonn,tabl
%O A055258 0,1
%A A055258 _Henry Bottomley_, Jun 22 2000