|
%I #21 Jan 02 2020 04:16:00
%S 3280,6160,8320,51520,99904,174640,386320,541840,883120,1690960,
%T 2062480,2365312,2688880,2959744,3077680,3152560,3274240,5375920,
%U 6885760,8925312,10030720,11219584,11912080,12058960,14370160,15854080,18966640,21839440,22038160,24787840,26725360
%N Refactorable numbers k such that 2*k + 1 is also a refactorable number.
%C If k is in this sequence, then 2*k + 1 must be a square. So this sequence is a subsequence of A046092.
%H Amiram Eldar, <a href="/A281294/b281294.txt">Table of n, a(n) for n = 1..10000</a>
%e 3280 is a term because 3280 = 2^4 * 5 * 41 is divisible by d(3280) = 2^2 * 5 and 2 * 3280 + 1 = 3^8 is divisible by d(3^8) = 9.
%t Select[Table[2 n (n + 1), {n, 10^4}], Times @@ Boole@ Thread[Divisible[#, DivisorSigma[0, #]] &@ {#, 2 # + 1}] > 0 &] (* _Michael De Vlieger_, Jan 19 2017 *)
%o (PARI) isA033950(n) = n % numdiv(n)== 0;
%o is(n) = isA033950(n) && isA033950(2*n+1);
%Y Cf. A033950, A036896, A046092.
%K nonn
%O 1,1
%A _Altug Alkan_, Jan 19 2017
|
