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A281294 Refactorable numbers k such that 2*k + 1 is also a refactorable number. 2
3280, 6160, 8320, 51520, 99904, 174640, 386320, 541840, 883120, 1690960, 2062480, 2365312, 2688880, 2959744, 3077680, 3152560, 3274240, 5375920, 6885760, 8925312, 10030720, 11219584, 11912080, 12058960, 14370160, 15854080, 18966640, 21839440, 22038160, 24787840, 26725360 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If k is in this sequence, then 2*k + 1 must be a square. So this sequence is a subsequence of A046092.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

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.

MATHEMATICA

Select[Table[2 n (n + 1), {n, 10^4}], Times @@ Boole@ Thread[Divisible[#, DivisorSigma[0, #]] &@ {#, 2 # + 1}] > 0 &] (* Michael De Vlieger, Jan 19 2017 *)

PROG

(PARI) isA033950(n) = n % numdiv(n)== 0;

is(n) = isA033950(n) && isA033950(2*n+1);

CROSSREFS

Cf. A033950, A036896, A046092.

Sequence in context: A309284 A015326 A252074 * A043472 A203859 A107536

Adjacent sequences:  A281291 A281292 A281293 * A281295 A281296 A281297

KEYWORD

nonn

AUTHOR

Altug Alkan, Jan 19 2017

STATUS

approved

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Last modified August 17 03:53 EDT 2022. Contains 356184 sequences. (Running on oeis4.)