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 A359500 #18 Feb 10 2025 20:49:10
%S A359500 3,75,90075,259632270075,4314170602515315024630075,
%T A359500 2382344702413741601833152075318304337413311121350075,
%U A359500 1452944967966417671787414728262962471027692106596483349510252251060925112718067382475349181570930962790075
%N A359500 a(n) = (7^2^n - 1)/2^(n+3).
%H A359500 Jianing Song, <a href="/A359500/b359500.txt">Table of n, a(n) for n = 1..10</a>
%F A359500 a(n)/a(n-1) = (7^2^(n-1) + 1)/2 = 2^(n+1) * a(n-1) + 1.
%F A359500 For all n > m, a(n) and a(m) are congruent modulo 2^(m+2) but not modulo 2^(m+3).
%o A359500 (PARI) a(n) = (7^2^n - 1)/2^(n+3)
%o A359500 (Python)
%o A359500 def A359500(n): return 7**(1<<n)-1>>n+3 # _Chai Wah Wu_, Jan 15 2023
%Y A359500 A261066 is twice this sequence.
%Y A359500 Cf. {odd part of m^2^n - 1}: A068531 (m=3 or m=9), A097421 (m=5), this sequence (m=7).
%K A359500 nonn,easy
%O A359500 1,1
%A A359500 _Jianing Song_, Jan 03 2023