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.
The third term of A066549 is 711 hence a(3) = 7.
With[{c=Flatten[IntegerDigits/@(Range[100]^2)]},Table[FromDigits[Take[c,{(n(n-1))/2+1,(n(n-1))/2+n}]],{n,20}]] (* Harvey P. Dale, Jul 21 2012 *)
from itertools import count, islice
def agen(): # generator of terms
N, i = [], 1
for n in count(1):
target = n*(n-1)//2
while len(N) <= target+n+1:
N.extend(list(str(i**2)))
i += 1
yield int("".join(N[target:target+n]))
print(list(islice(agen(), 21))) # Michael S. Branicky, May 23 2025
With[{c=Flatten[IntegerDigits/@(Range[400]^2)]},Table[Total[Take[c,{(n(n-1))/2+1,(n(n-1))/2+n}]],{n,60}]] (* Harvey P. Dale, Jul 21 2012 *)
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