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.
a(0) = 1 = 1 a(1) = 1 + 2 = 3 a(2) = 1 + 2 - 3 = 0 a(3) = 1 + 2 - 3 - 4 = -4 a(4) = 1 + 2 - 3 - 4 + 5 = 1 a(5) = 1 + 2 - 3 - 4 + 5 + 6 = 7 a(6) = 1 + 2 - 3 - 4 + 5 + 6 + 7 = 14
a:= proc(n) option remember: `if`(n=0, 1,
a(n-1)+(n+1)*(-1)^floor(sqrt(2*n)-1/2))
end:
seq(a(n), n=0..60); # Alois P. Heinz, Feb 26 2019
With[{nn=20},Accumulate[Flatten[Join[{1,2},Times@@@Partition[Riffle[TakeList[Range[3,3+(nn(nn+1))/2],Range[2,nn]],{-1,1}],2]]]]] (* Harvey P. Dale, Mar 24 2024 *)
from math import isqrt def A306562(n): return 1+sum((k if isqrt(k-1<<3)+1&2 else -k) for k in range(2,n+2)) # Chai Wah Wu, Jun 07 2025