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 A180245 #10 Feb 20 2023 07:50:56 %S A180245 3,33,42,196,918,6640,24750,246078,781248,6565374,25227774,165009150, %T A180245 673932798,5268548608,25737162750,179511912448,818179991550, %U A180245 4228689854464,26455088693248,104384041582590,820632501420030 %N A180245 n-th natural number m such that m and m+2 are both divisible by exactly n primes (counted with multiplicity). %C A180245 Main diagonal A[n,n] of A[k,n] = n-th natural number m such that m and m+2 are both divisible by exactly k primes (counted with multiplicity). %C A180245 This is the main diagonal of the array mentioned in A180117, A180150, and A180151. %C A180245 Row 1 = A001359 = the lesser of twin primes. %C A180245 Row 2 = A092207 = Numbers n such that n and n+2 are semiprimes. %C A180245 Row 3 = A180117 = m and m+2 are both divisible by exactly 3 primes (counted with multiplicity). %C A180245 Row 4 = A180150 = m and m+2 are both divisible by exactly 4 primes (counted with multiplicity). %C A180245 Row 5 = A180151 = m and m+2 are both divisible by exactly 5 primes (counted with multiplicity). %e A180245 a(1) = 3 because 3 is the first natural number m such that m and m+2 are both divisible by exactly 1 prime (i.e., the first of the lesser of twin primes). %e A180245 a(2) = 33 because that is the 2nd natural number m such that m and m+2 are both divisible by exactly 2 primes (i.e. 33 = 3 * 11 is semiprime and when 2 is added becomes 35 = 5 * 7 which is also semiprimes) the 1st such being 4. %Y A180245 Cf. A001359, A092207, A180117, A180150, A180151. %K A180245 nonn %O A180245 1,1 %A A180245 _Jonathan Vos Post_, Aug 19 2010 %E A180245 Corrected and extended by _Jack Brennen_, _D. S. McNeil_ and _Ray Chandler_, Aug 19 2010 %E A180245 a(16)-a(21) from _Donovan Johnson_, Aug 27 2010