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 A055237 #26 Apr 08 2025 13:57:44
%S A055237 2,6,10,26,30,50,126,130,150,250,626,630,650,750,1250,3126,3130,3150,
%T A055237 3250,3750,6250,15626,15630,15650,15750,16250,18750,31250,78126,78130,
%U A055237 78150,78250,78750,81250,93750,156250,390626,390630,390650,390750,391250,393750,406250,468750,781250
%N A055237 Sums of two powers of 5.
%H A055237 T. D. Noe, <a href="/A055237/b055237.txt">Rows n = 0..100 of triangle, flattened</a>
%H A055237 Michael Penn, <a href="https://www.youtube.com/watch?v=pIk48dyiJWM">Fives and squares.</a>, YouTube video, 2021.
%F A055237 a(n) = 5^(n-trinv(n))+5^trinv(n), where trinv(n) = floor((1+sqrt(1+8*n))/2) = A002262(n) and n-trinv(n) = A003056(n).
%F A055237 Regarded as a triangle T(n, k) = 5^n + 5^k, so as a sequence a(n) = 5^A002262(n) + 5^A003056(n).
%t A055237 t = 5^Range[0, 9]; Select[Union[Flatten[Table[i + j, {i, t}, {j, t}]]], # <= t[[-1]] + 1 &] (* _T. D. Noe_, Oct 09 2011 *)
%t A055237 Total/@Tuples[5^Range[0,9],2]//Union (* _Harvey P. Dale_, Jan 29 2017 *)
%o A055237 (Python)
%o A055237 from math import isqrt
%o A055237 def A055237(n): return 5**(a:=(k:=isqrt(m:=n<<1))+(m>k*(k+1))-1)+5**(n-1-(a*(a+1)>>1)) # _Chai Wah Wu_, Apr 08 2025
%Y A055237 Cf. A052216.
%K A055237 nonn,easy,tabl
%O A055237 0,1
%A A055237 _Henry Bottomley_, Jun 22 2000