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 A073247 #28 Nov 27 2024 07:21:16
%S A073247 17,19,26,51,53,55,89,91,97,127,149,151,161,163,170,197,199,233,235,
%T A073247 241,249,251,269,271,293,295,305,307,337,339,341,349,362,377,379,413,
%U A073247 415,449,451,485,487,489,491,521,523,530,551,557,559,577,579,593,595
%N A073247 Squarefree numbers k such that k-1 and k+1 are not squarefree.
%C A073247 Probably 11*n < a(n) < 12*n for n > 189. - _Charles R Greathouse IV_, Nov 05 2017
%C A073247 The asymptotic density of this sequence is 1/zeta(2) - 2 * Product_{p prime} (1 - 2/p^2) + Product_{p prime} (1 - 3/p^2) = A059956 - 2*A065474 + A206256 = 0.088145884881346585838... . - _Amiram Eldar_, Aug 30 2024
%H A073247 Christopher E. Thompson, <a href="/A073247/b073247.txt">Table of n, a(n) for n = 1..10000</a>
%p A073247 sf:= select(numtheory:-issqrfree,[$1..1000]):
%p A073247 map(t -> `if`(sf[t-1]=sf[t]-1 or sf[t+1]=sf[t]+1,NULL,sf[t]), [$2..nops(sf)-1]); # _Robert Israel_, Feb 01 2016
%t A073247 Reap[For[n = 0, n <= 1000, n++, If[SquareFreeQ[n] && !SquareFreeQ[n-1] && !SquareFreeQ[n+1], Sow[n]]]][[2, 1]] (* _Jean-François Alcover_, Feb 26 2019 *)
%o A073247 (PARI) is(n)=!issquarefree(n-1) && issquarefree(n) && !issquarefree(n+1) \\ _Charles R Greathouse IV_, Nov 05 2017
%o A073247 (PARI) list(lim)=my(v=List(),l1,l2); forfactored(k=9,lim\1+1, if(!issquarefree(k) && !issquarefree(l2) && issquarefree(l1), listput(v,l1[1])); l2=l1; l1=k); Vec(v) \\ _Charles R Greathouse IV_, Nov 27 2024
%Y A073247 Cf. A005117, A073249, A073248, A007675.
%Y A073247 Cf. A268331, A268332, A268333, A268334 (squarefree numbers isolated by more than 2, 3, etc.).
%Y A073247 Cf. A059956, A065474, A206256.
%K A073247 nonn
%O A073247 1,1
%A A073247 _Reinhard Zumkeller_, Jul 22 2002