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 A295043 a(n) is the largest number k such that sigma(k) = 2^n or 0 if no such k exists. 1
 1, 0, 3, 7, 0, 31, 0, 127, 217, 381, 889, 0, 3937, 8191, 11811, 27559, 57337, 131071, 253921, 524287, 1040257, 1777447, 4063201, 7281799, 16646017, 32247967, 66584449, 116522119, 225735769, 516026527, 1073602561, 2147483647, 4294434817, 7515217927, 15032385529 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS If a(n) > 0, then it is a term of A046528 (numbers that are a product of distinct Mersenne primes). LINKS FORMULA a(A078426(n)) = 0. a(A180221(n)) > 0. a(n) <= 2^n - 1 with equality when n is a Mersenne exponent (A000043). - Michael B. Porter, Nov 14 2017 EXAMPLE a(0) = 1 because 1 is the largest number k with sigma(k) = 1 = 2^0. a(5) = 31 because 31 is the largest number k with sigma(k) = 32 = 2^5. a(6) = 0 because there is no number k with sigma(k) = 64 = 2^6. PROG (PARI) a(n) = {local(r, k); r=0; for(k=1, 2^n, if(sigma(k) == 2^n, r=k)); return(r)}; \\ Michael B. Porter, Nov 14 2017 (PARI) a(n) = forstep(k=2^n, 1, -1, if (sigma(k)==2^n, return (k))); return (0) \\ Rémy Sigrist, Jan 08 2018 CROSSREFS Cf. A000043, A000203, A046528, A048947, A057637, A180221. Cf. A247956 (the smallest number k instead of the largest). Cf. A078426 (no solution to the equation sigma(x)=2^n). A000668 (Mersenne primes) is a subsequence. Sequence in context: A199068 A198490 A247956 * A074051 A048292 A072450 Adjacent sequences:  A295040 A295041 A295042 * A295044 A295045 A295046 KEYWORD nonn AUTHOR Jaroslav Krizek, Nov 13 2017 STATUS approved

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Last modified May 25 15:19 EDT 2019. Contains 323570 sequences. (Running on oeis4.)