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58 | 58 |
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59 | 59 | - The best known **explicit** exponent separating sensitivity from degree is currently $\log_3 6$ (the “Kushilevitz barrier”). Improving this exponent is an open problem. “Kushilevitz function” was introduced (unpublished by Kushilevitz) in **Footnote 1 of Nisan and Wigderson’s paper** [NW95]. |
60 | 60 |
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61 | | -- (Kushilevitz base.) One explicit polynomial representing the base function |
| 61 | +- (Kushilevitz function.) One explicit polynomial representing the Kushilevitz function |
62 | 62 | $$ |
63 | 63 | h:\{0,1\}^6\to\{0,1\} |
64 | 64 | $$ |
65 | 65 | is |
66 | 66 | $h(z_1,\dots,z_6)=\sum_{i=1}^6 z_i - \sum_{1\le i<j\le 6} z_i z_j + (z_1z_3z_4 + z_1z_2z_5 + z_1z_4z_5 + z_2z_3z_4 + z_2z_3z_5 + z_1z_2z_6 + z_1z_3z_6 + z_2z_4z_6 + z_3z_5z_6 + z_4z_5z_6),$ |
67 | 67 |
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68 | | - which is Boolean on $\{0,1\}^6$, has degree $3$, and is fully sensitive at $(0,0,0,0,0,0)$. |
| 68 | + which is Boolean on {0,1}^6, has degree $3$, and max sensitivity $s(f)=6$ achieving at $x=(0,0,0,0,0,0)$. |
69 | 69 |
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70 | 70 |
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71 | | -- Before Kushilevitz’s 6-variable base (giving exponent $\log_3 6$), a simpler 3-variable base already yields exponent $\log_2 3$ via self-composition. One concrete choice is the "not-all-equal" function on 3 bits, |
| 71 | +- Before Kushilevitz’s 6-variable function (giving exponent $\log_3 6$), a simpler 3-variable function already yields exponent $\log_2 3$. One concrete choice is the "not-all-equal" function on 3 bits, |
72 | 72 |
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73 | 73 | $g(x_1,x_2,x_3)=x_1+x_2+x_3-x_1x_2-x_1x_3-x_2x_3,$ |
74 | 74 |
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75 | | - which is Boolean on the cube, has $\deg(g)=2$, and has full sensitivity $s(g)=3$ at $(0,0,0)$. Composing $g$ with itself $k$ times multiplies degree and (pointwise) sensitivity at $0$, yielding $s=3^k$, $\deg=2^k$. |
| 75 | + which is Boolean on the cube, has $\deg(g)=2$, and $s(g)=3$ at $x=(0,0,0)$. |
76 | 76 |
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77 | 77 | - For background on the general relationship between sensitivity, block sensitivity, and degree (including Huang’s proof of the Sensitivity Conjecture), see [H2019] and the surveys [BdW2002], [HKP2011]. |
78 | 78 |
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