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 A380233 #40 Jan 18 2025 09:35:38
%S A380233 153153,171171,189189,207207,243243,261261,279279,297297,351351,
%T A380233 459459,513513,567567,621621,729729,783783,837837,891891,999999,
%U A380233 1392313923,1556115561,1719917199,1883718837,2034920349,2211322113,2375123751,2538925389,2702727027,3194131941,4176941769,4668346683
%N A380233 Odd abundant numbers not divisible by 5 that are also doublets (cf. A020338).
%C A380233 There are 26 odd abundant numbers not divisible by 5 less than 10^6. The surprising fact is that 18 of them are doublets.
%C A380233 Another interesting fact is that here there are no terms with 8 digits.
%p A380233 doublet:= n -> n * (10^(1+ilog10(n))+1):
%p A380233 select(t -> numtheory:-sigma(t) > 2*t, [seq(seq(doublet(10*x+i),i=[1,3,7,9]),x=1..10000); # _Robert Israel_, Jan 17 2025
%t A380233 Select[Table[FromDigits[Join[#, #] &@ IntegerDigits[n]], {n, Select[Range[50000], CoprimeQ[#, 10] &]}], DivisorSigma[-1, #] > 2 &] (* _Amiram Eldar_, Jan 17 2025 *)
%o A380233 (PARI) select(x->((x%5) && (sigma(x)>2*x)), vector(50000, n, eval(Str(2*n-1, 2*n-1)))) \\ _Michel Marcus_, Jan 17 2025
%Y A380233 Intersection of A064001 and A020338.
%Y A380233 Cf. A005101, A005231, A047201.
%K A380233 nonn,base
%O A380233 1,1
%A A380233 _Omar E. Pol_, Jan 17 2025
%E A380233 More terms from _Michel Marcus_, Jan 17 2025