A247956 a(n) is the smallest number k such that sigma(k) = 2^n or 0 if no such k exists.
1, 0, 3, 7, 0, 21, 0, 93, 217, 381, 651, 0, 2667, 8191, 11811, 24573, 57337, 82677, 172011, 393213, 761763, 1572861, 2752491, 5332341, 11010027, 21845397, 48758691, 85327221, 199753347, 341310837, 677207307, 1398273429, 3220807683, 6192353757, 10836557067
Offset: 0
Keywords
Examples
a(0) = 1 because 1 is the smallest number k with sigma(1) = 1 = 2^0. a(5) = 21 because 21 is the smallest number k with sigma(k) = 32 = 2^5. a(6) = 0 because there is no number k with sigma(k) = 64 = 2^6.
Links
- Amiram Eldar, Table of n, a(n) for n = 0..300
- Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).
Programs
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PARI
a(n) = for (k=1, 2^n, if (sigma(k)== 2^n, return (k))); return (0); \\ Michel Marcus, Oct 03 2014, Oct 31 2015
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PARI
a(n) = max(0, invsigmaMin(1<
Extensions
a(0) = 1 prepended by Michel Marcus, Oct 31 2015
Comments