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 A155875 #6 Apr 09 2021 14:41:48 %S A155875 4,9,6,9,8,9,10,15,12,15,14,15,16,21,18,21,20,21,22,25,24,25,26,27,28, %T A155875 33,30,33,32,33,34,35,36,39,38,39,40,45,42,45,44,45,46,49,48,49,50,51, %U A155875 52,55,54,55,56,57,58,63,60,63,62,63,64,65,66,69,68,69,70 %N A155875 Smallest positive composite number such that a(n) - n is also composite. %C A155875 a(0) = 4. Subtracting n = 0 from a(0) gives 4-0 = 4, which is a composite number; subtracting n = 1 from a(1) gives 9-1 = 8 which is composite; subtracting n = 2 from a(2) gives 6-2 = 4 which is composite; subtracting n = 3 from a(3) gives 9-3 = 6 which is composite; etc. %o A155875 (Python) %o A155875 from sympy import isprime %o A155875 def composite(n): return n >= 4 and not isprime(n) %o A155875 def a(n): %o A155875 an = n + 4 %o A155875 while not (composite(an) and composite(an-n)): an += 1 %o A155875 return an %o A155875 print([a(n) for n in range(67)]) # _Michael S. Branicky_, Apr 09 2021 %Y A155875 Cf. A155874, A122985. %K A155875 base,easy,nonn %O A155875 0,1 %A A155875 _Eric Angelini_, Jan 29 2009 %E A155875 More terms from _Michael S. Branicky_, Apr 09 2021