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20090503 hardnessvsrandomness itsykson_lecture10
1.
Âû÷èñëèòåëüíî òðóäíûå çàäà÷è
è äåðàíäîìèçàöèÿ Ëåêöèÿ 10: Ýêñòðàêòîðû Äìèòðèé Èöûêñîí ÏÎÌÈ ÐÀÍ 3 ìàÿ 2009 1/9
2.
Ïëàí 1 Ñëàáûå èñòî÷íèêè
ñëó÷àéíûõ áèòîâ 2 Ýêñòðàêòîðû 3 Ñóùåñòâîâàíèå ýêñòðàêòîðîâ 4 Êîíñòðóêöèÿ ýêñòðàêòîðà, îñíîâàííàÿ íà ñëó÷àéíîì áëóæäàíèè ïî ýêñïàíäåðó 2/9
3.
Ïîñòàíîâêà çàäà÷è •
Ïóñòü àëãîðèòìó òðåáóåòñÿ k ñëó÷àéíûõ áèòîâ. • Äîñòàòî÷íî èìåòü ðàâíîìåðíîå ðàñïðåäåëåíèå íà {0, 1}k . • Èëè ïî÷òè ðàâíîìåðíîå... • Ïóñòü D1 , D2 ðàñïðåäåëåíèÿ íà {0, 1}n . Ñòàòèñòè÷åñêîå ðàññòîÿíèå: δ(D1 , D2 ) = x∈{0,1}n |D1 (x) − D2 (x)|. • 1 Äîñòàòî÷íî èìåòü ðàñïðåäåëåíèå íà ðàññòîÿíèè 10 îò ðàâíîìåðíîãî 3/9
4.
Ñëàáûå èñòî÷íèêè ñëó÷àéíûõ
áèòîâ • Ïóñòü D ðàñïðåäåëåíèå íà {0, 1}n . • Ìèíèìàëüíàÿ ýíòðîïèÿ H∞ (D) = max{k ∈ R | ∀z D(z) ≤ 2−k }. • 0 ≤ H∞ (D) ≤ n; åñëè H∞ (D) = n, òî X ðàâíîìåðíîå. • H∞ (D) ýòî ÷èñëî íàñòîÿùèõ ñëó÷àéíûõ áèòîâ, êîòîðûå ñîäåðæèò ðàñïðåäåëåíèå D. • D Ðàñïðåäåëåíèå n íà {0, 1} ýòî (n, k)-èñòî÷íèê, åñëè, H∞ (D) ≥ k ⇐⇒ ∀z D(z) ≤ 2−k . 4/9
5.
Ïðèìåðû ðàñïðåäåëåíèé 1 (Çàôèêñèðîâàííûå
áèòû) S ⊆ [n], |S| = k . Ðàñïðåäåëåíèå ðàâíîìåðíîå íà áèòàõ èç S è ôèêñèðîâàííàÿ ñòðî÷êà íà áèòàõ èç [n] S . H∞ = k . 2 (Ëèíåéíîå ïîäïðîñòðàíñòâî) Ðàâíîìåðíîå ðàñïðåäåëåíèå íà ëèíåéíîì ïîäïðîñòðàíñòâå Zn 2 ðàçìåðíîñòè k . H∞ = k . 3 (Ðàâíîìåðíîå íà ìíîæåñòâå) S⊆ {0, 1}n , |S| = 2k , ðàñïðåäåëåíèå ðàâíîìåðíîå íà S . H∞ = k . 4 (Íåñèììåòðè÷íàÿ ìîíåòà) n ðàç íåçàâèñèìî áðîñàåòñÿ 1 ìîíåòà, êîòîðàÿ âûäàåò 1 ñ âåðîÿòíîñòüþ δ 2 è 0 ñ 1 âåðîÿòíîñòüþ 1 − δ . H∞ = log 1−δ n 5/9
6.
Îïðåäåëåíèå ýêñòðàêòîðà Îïðåäåëåíèå. Ôóíêöèÿ
Ext : {0, 1}n × {0, 1}d → {0, 1}m íàçûâàåòñÿ (k, )-ýêñòðàêòîðîì, åñëè äëÿ ëþáîãî (n, k)-èñòî÷íèêà D , ðàñïðåäåëåíèå Ext(D, Ud ) íàõîäèòñÿ íà ðàññòîÿíèè ≤ îò Um . • Çà÷åì äîïîëíèòåëüíûå íàñòîÿùèå ñëó÷àéíûå áèòû? • Ext : {0, 1}n → {0, 1}m • Ïóñòü | Ext−1 (1∗)| ≥ 2n−1 , D ðàâíîìåðíîå ðàñïðåäåëåíèå íà íà Ext−1 (1∗), H∞ (D) = n − 1. Ext(D) íàõîäèòñÿ íà 1 ðàññòîÿíèè õîòÿ áû 2 îò ðàâíîìåðíîãî. • Åñëè d = O(log n), òî âñå ñòðîêè äëèíû d ìîæíî ïåðåáðàòü. 6/9
7.
Ñóùåñòâîâàíèå ýêñòðàêòîðà Òåîðåìà.
∀k, n, ñóùåñòâóåò (k, )-ýêñòðàêòîð Ext : {0, 1}n × {0, 1}d → {0, 1}k , ãäå d = log n + 2 log(1/ ) + O(1). Äîêàçàòåëüñòâî. • (n, k)-èñòî÷íèê D ïëîñêèé, åñëè D ýòî ðàâíîìåðíîå ðàñïðåäåëåíèå íà ìíîæåñòâå ðàçìåðà 2k . • Ëþáîé (n, k)-èñòî÷íèê ýòî âûïóêëàÿ êîìáèíàöèÿ ïëîñêèõ. Äîñòàòî÷íî äîêàçûâàòü òîëüêî äëÿ ïëîñêèõ (n, k)-èñòî÷íèêîâ. • Âûáåðåì Ext : {0, 1}n × {0, 1}d → {0, 1}k ñëó÷àéíûì îáðàçîì. • Ïóñòü D ýòî ïëîñêèé (n, k)-èñòî÷íèê ñ íîñèòåëåì S , |S| = 2k . • f : {0, 1}k → {0, 1} íåêîòîðàÿ ôóíêöèÿ. 1 • E[f (Ext(D, Ud ))] = 2k ×2d x∈S,y ∈{0,1}d f (Ext(x, y )). 7/9
8.
Ñóùåñòâîâàíèå ýêñòðàêòîðà •
Ïóñòü D ýòî ïëîñêèé (n, k)-èñòî÷íèê ñ íîñèòåëåì S, |S| = 2k . • f : {0, 1}k → {0, 1} íåêîòîðàÿ ôóíêöèÿ. • E[f (Ext(D, Ud ))] = 1 2k ×2d x∈S,y ∈{0,1}d f (Ext(x, y )). • Âû÷èñëèòü E[f (Ext(D, Ud ))] ýòî òîæå ñàìîå, ÷òî 2k × 2d ðàç k çàïóñòèòü f íà ñëó÷àéíîé ñòðîêå èç {0, 1} . • Èç îöåíîê ×åðíîâà E[f (Ext(D, Ud ))] îòëè÷àåòñÿ îò k+d 2 /4 E[f (Uk )] íà 2 ñ âåðîÿòíîñòüþ ìåíüøå, ÷åì 2−2 • Ïðè d log n + 2 log(1/ ) + 3 ýòà âåðîÿòíîñòü ìåíüøå, ÷åì k k 2−(2n)2 . ×èñëî ïëîñêèõ èñòî÷íèêîâ íå áîëüøå, ÷åì (2n )2 , k ÷èëî ôóíêöèé {0, 1} → {0, 1} ðîâíî 2 . 2k • Íàéäåòñÿ òàêîé Ext, ÷òî äëÿ âñåõ f è D | E[f (Ext(D, Ud ))] − E[f (Uk )]| 2 8/9
9.
Ýêñòðàêòîð èç ñëó÷àéíîãî
áëóæäàíèÿ ïî ýêñïàíäåðó Ëåììà. ∀ 0, n ≥ k ìîæíî ïîñòðîèòü (k, )-ýêñòðàêòîð Ext : {0, 1}n × {0, 1}t → {0, 1}n , ãäå t = O(n − k + log 1 ). Äîêàçàòåëüñòâî. • Ïóñòü A íîðìèðîâàííàÿ ìàòðèöà ñìåæíîñòè (2n , d, 1 )-àëãåáðàè÷åñêîãî ðàñøèðèòåëÿ. D ýòî 2 (n, k)-èñòî÷íèê. • t áèòîâ îïèñûâàþò ñëó÷àéíîå áëóæäàíèå. • Am D ðàñïðåäåëåíèå ïîñëå m øàãîâ áëóæäàíèÿ. • Am D− 2n 1 1 ≤ 2n/2 Am (D − 2n 1) 2 ≤ 2n/2 21 D − 2n 1 2 1 1 m 1 1 1 1 • D − 2n 1 2 = x (D(x) − 2n )2 = x (D(x))2 + 2n − 2 x D(x) 2n ≤ x (D(x))2 ≤ 2k · 2−2k = 2−k 1 (n−k)/2 • Ak D − 2n 1 1 ≤ 2 2m 1 1 • Âûáåðåì m = n − k + log 9/9
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