Explicit construction For s = 1, C 1 can be the identity matrix, which generates the famous digital (0 , 1)-sequence called van der Corput sequence . For s = 2, C 1 and C 2 can be 1 0 · · · 0 0 · · · 0 1 0 1 · · · 0 0 · · · 1 0 C 1 = , C 2 = , . . . . . . ... ... . . . . . . . . . . . . 0 0 · · · 1 1 · · · 0 0 which generates a digital (0 , m , 2)-net, known as the Hammersley point set. Not extensible in m . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 15 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Figure: Hammersley point set for m = 6 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 16 / 61
Explicit construction Let p 1 , p 2 , . . . ∈ F 2 [ x ] be a sequence of distinct primitive/irreducible polynomials over F 2 with e 1 ≤ e 2 ≤ · · · where e j = deg( p j ). Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 17 / 61
Explicit construction Let p 1 , p 2 , . . . ∈ F 2 [ x ] be a sequence of distinct primitive/irreducible polynomials over F 2 with e 1 ≤ e 2 ≤ · · · where e j = deg( p j ). For each j , C j = ( c ( j ) k , l ) is given by the coefficients of the following Laurent series: c ( j ) c ( j ) x e j − 1 1 , 1 1 , 2 p j ( x ) = + x 2 + · · · x . . . c ( j ) c ( j ) 1 e j , 1 e j , 2 p j ( x ) = + x 2 + · · · x c ( j ) c ( j ) x e j − 1 e j +1 , 1 e j +1 , 2 ( p j ( x )) 2 = + + · · · x 2 x . . . (Sobol’ 1967; Niederreiter 1988; Tezuka 1993) Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 17 / 61
Explicit construction All matrices C j ∈ F N × N are upper triangular and generate a digital 2 ( t , s )-sequence with t = ( e 1 − 1) + · · · + ( e s − 1) . (I used a C implementation for this sequence in at least more than 58636 dimensions due to Tomohiko Hironaka.) Figure: 2D projections of the first 2 6 points of the Niederreiter sequence. Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 18 / 61
Polynomial lattice point sets (Niederreiter, 1992) Let p ∈ F 2 [ x ] be irreducible with deg( p ) = m Let q = ( q 1 , . . . , q s ) ∈ ( F 2 [ x ]) s with deg( q j ) < m . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 19 / 61
Polynomial lattice point sets (Niederreiter, 1992) Let p ∈ F 2 [ x ] be irreducible with deg( p ) = m Let q = ( q 1 , . . . , q s ) ∈ ( F 2 [ x ]) s with deg( q j ) < m . For each j , C j is given by the square Hankel matrix a ( j ) a ( j ) a ( j ) · · · m 1 2 a ( j ) a ( j ) a ( j ) . . . 2 3 m +1 ∈ F m × m C j = . . . ... 2 . . . . . . a ( j ) a ( j ) a ( j ) . . . m m +1 2 m − 1 where a ( j ) 1 , a ( j ) 2 , . . . are the coefficients of the Laurent series p ( x ) = a ( j ) + a ( j ) q j ( x ) 1 2 x 2 + · · · . x Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 19 / 61
Polynomial lattice point sets (Niederreiter, 1992) Let p ∈ F 2 [ x ] be irreducible with deg( p ) = m Let q = ( q 1 , . . . , q s ) ∈ ( F 2 [ x ]) s with deg( q j ) < m . For each j , C j is given by the square Hankel matrix a ( j ) a ( j ) a ( j ) · · · m 1 2 a ( j ) a ( j ) a ( j ) . . . 2 3 m +1 ∈ F m × m C j = . . . ... 2 . . . . . . a ( j ) a ( j ) a ( j ) . . . m m +1 2 m − 1 where a ( j ) 1 , a ( j ) 2 , . . . are the coefficients of the Laurent series p ( x ) = a ( j ) + a ( j ) q j ( x ) 1 2 x 2 + · · · . x The resulting digital net is called a polynomial lattice point set P ( p , q ). Usually the vector q is constructed by a (fast) computer search algorithm (Nuyens & Cools, 2006; Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 19 / 61
Polynomial lattice point sets (Niederreiter, 1992) Let p ∈ F 2 [ x ] be irreducible with deg( p ) = m Let q = ( q 1 , . . . , q s ) ∈ ( F 2 [ x ]) s with deg( q j ) < m . For each j , C j is given by the square Hankel matrix a ( j ) a ( j ) a ( j ) · · · m 1 2 a ( j ) a ( j ) a ( j ) . . . 2 3 m +1 ∈ F m × m C j = . . . ... 2 . . . . . . a ( j ) a ( j ) a ( j ) . . . m m +1 2 m − 1 where a ( j ) 1 , a ( j ) 2 , . . . are the coefficients of the Laurent series p ( x ) = a ( j ) + a ( j ) q j ( x ) 1 2 x 2 + · · · . x The resulting digital net is called a polynomial lattice point set P ( p , q ). Usually the vector q is constructed by a (fast) computer search algorithm (Nuyens & Cools, 2006; P. Kritzer, this afternoon!). Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 19 / 61
Outline QMC and digital nets/sequences Classical QMC Higher order QMC Richardson extrapolation and QMC Application 1: Truncation of higher order nets and sequences Application 2: Extrapolated polynomial lattice rules Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 20 / 61
What if f is smooth In some applications, such as PDEs with random coefficients, f can be smooth. A proper design of QMC point sets enables higher order convergence of the integration error than O (1 / N ) as expected from the KH inequality. So far, QMC point sets achieving higher order convergence for non-periodic smooth functions are 1 Higher order digital nets/sequences (Dick, 2008; ...), and 2 Tent-transformed lattice point sets (Hickernell, 2002; ...). Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 21 / 61
What if f is smooth In some applications, such as PDEs with random coefficients, f can be smooth. A proper design of QMC point sets enables higher order convergence of the integration error than O (1 / N ) as expected from the KH inequality. So far, QMC point sets achieving higher order convergence for non-periodic smooth functions are 1 Higher order digital nets/sequences (Dick, 2008; ...), and 2 Tent-transformed lattice point sets (Hickernell, 2002; ...). In this talk, I will focus on higher order digital nets/sequences . The contents of this section are mostly developed by Dick (2008). Please refer to a recent review by G. & Suzuki (arXiv:1903.12353). Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 21 / 61
Quality measure: high order t -value Let α ∈ N . Let t be an integer such that, for any choice 1 ≤ d j , v j < · · · < d j , 1 ≤ α m , 0 ≤ v j ≤ α m , 1 ≤ j ≤ s , with min( v j ,α ) s ∑ ∑ d j , i = α m − t , j =1 i =1 the d 1 , v 1 , . . . , d 1 , 1 -th rows of C 1 . . . the d s , v s , . . . , d s , 1 -th rows of C s are linearly independent over F 2 . P is called an order α digital ( t , m , s ) -net . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 22 / 61
Quality measure: high order t -value Order α digital ( t , m , s )-nets hold equi-distribution properties: union of dyadic elementary boxes contains the fair number of points (shown later visually). Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 23 / 61
Quality measure: high order t -value Order α digital ( t , m , s )-nets hold equi-distribution properties: union of dyadic elementary boxes contains the fair number of points (shown later visually). Let t be an integer such that, for any α m ≥ t , the first 2 m points of S are an order α digital ( t , m , s )-net. S is called an order α digital ( t , s ) -sequence . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 23 / 61
Explicit construction Define D α : [0 , 1) α → [0 , 1) by x 1 = (0 .ξ (1) 1 ξ (1) 2 ξ (1) . . . ) 2 3 x 2 = (0 .ξ (2) 1 ξ (2) 2 ξ (2) . . . ) 2 3 �→ (0 . ξ (1) 1 ξ (2) . . . ξ ( α ) ξ (1) 2 ξ (2) . . . ξ ( α ) . . . ) 2 . . 1 1 2 2 . . � �� � � �� � x α = (0 .ξ ( α ) ξ ( α ) ξ ( α ) α α . . . ) 2 1 2 3 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 24 / 61
Explicit construction Define D α : [0 , 1) α → [0 , 1) by x 1 = (0 .ξ (1) 1 ξ (1) 2 ξ (1) . . . ) 2 3 x 2 = (0 .ξ (2) 1 ξ (2) 2 ξ (2) . . . ) 2 3 �→ (0 . ξ (1) 1 ξ (2) . . . ξ ( α ) ξ (1) 2 ξ (2) . . . ξ ( α ) . . . ) 2 . . 1 1 2 2 . . � �� � � �� � x α = (0 .ξ ( α ) ξ ( α ) ξ ( α ) α α . . . ) 2 1 2 3 For x ∈ [0 , 1) α s , let D α ( x ) = ( D α ( x 1 , . . . , x α ) , D α ( x α +1 , . . . , x 2 α ) , . . . , D α ( x α ( s − 1)+1 , . . . , x α s )) ∈ [0 , 1) s . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 24 / 61
Explicit construction Define D α : [0 , 1) α → [0 , 1) by x 1 = (0 .ξ (1) 1 ξ (1) 2 ξ (1) . . . ) 2 3 x 2 = (0 .ξ (2) 1 ξ (2) 2 ξ (2) . . . ) 2 3 �→ (0 . ξ (1) 1 ξ (2) . . . ξ ( α ) ξ (1) 2 ξ (2) . . . ξ ( α ) . . . ) 2 . . 1 1 2 2 . . � �� � � �� � x α = (0 .ξ ( α ) ξ ( α ) ξ ( α ) α α . . . ) 2 1 2 3 For x ∈ [0 , 1) α s , let D α ( x ) = ( D α ( x 1 , . . . , x α ) , D α ( x α +1 , . . . , x 2 α ) , . . . , D α ( x α ( s − 1)+1 , . . . , x α s )) ∈ [0 , 1) s . For a digital ( t , m , α s )-net P , we write D α ( P ) = {D α ( x ) | x ∈ P } . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 24 / 61
Another look at construction Let P be a digital ( t , m , α s )-net with C 1 , . . . , C α s ∈ F m × m . We write 2 c (1) c (2) c ( α ) 1 1 1 . . . . . . C 1 = , C 2 = , . . . , C α = , . . . . . . . c (1) c (2) c ( α ) m m m Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 25 / 61
Another look at construction Let P be a digital ( t , m , α s )-net with C 1 , . . . , C α s ∈ F m × m . We write 2 c (1) c (2) c ( α ) 1 1 1 . . . . . . C 1 = , C 2 = , . . . , C α = , . . . . . . . c (1) c (2) c ( α ) m m m D α ( P ) is a digital net with D 1 , . . . , D s ∈ F α m × m where 2 c (1) c ( α +1) 1 1 c (2) c ( α +2) 1 . 1 . . . . . c ( α ) c (2 α ) 1 1 . . . . D 1 = , D 2 = , . . . . . . c (1) c ( α +1) m m c (2) c ( α +2) m m . . . . . . c ( α ) c (2 α ) m m Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 25 / 61
High order t -value For a digital ( t , m , α s )-net P , D α ( P ) is an order α digital ( t ′ , m , s )-net with { ⌊ s ( α − 1) ⌋} t ′ ≤ α min m , t + . 2 For a digital ( t , α s )-sequences S , D α ( S ) is an order α digital ( t ′ , s )-sequences with t ′ ≤ α t + s α ( α − 1) . 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 26 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced Sobol’ point sets with α = 2 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 27 / 61
Interlaced polynomial lattice point sets Let p ∈ F 2 [ x ] be irreducible with deg( p ) = m , and let q = ( q 1 , . . . , q α s ) ∈ ( F 2 [ x ]) α s with deg( q j ) < m . An interlaced polynomial lattice point set is just D α ( P ( p , q )) . Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 28 / 61
Interlaced polynomial lattice point sets Let p ∈ F 2 [ x ] be irreducible with deg( p ) = m , and let q = ( q 1 , . . . , q α s ) ∈ ( F 2 [ x ]) α s with deg( q j ) < m . An interlaced polynomial lattice point set is just D α ( P ( p , q )) . The vector q can be constructed component by component. In the simplest case, the necessary construction cost is of O ( α sN log N ) with O ( N ) memory (G. & Dick, 2015; G., 2015). In applications to PDEs with random coefficients, the criterion sometimes becomes a bit complicated, requiring O ( α sN log N + α 2 s 2 N ) construction cost with O ( α sN ) memory (Dick, Kuo, Le Gia, Nuyens & Schwab, 2014). Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 28 / 61
Outline QMC and digital nets/sequences Classical QMC Higher order QMC Richardson extrapolation and QMC Application 1: Truncation of higher order nets and sequences Application 2: Extrapolated polynomial lattice rules Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 29 / 61
Walsh functions For k = ( . . . κ 1 κ 0 ) 2 ∈ N 0 , the k -th Walsh function is defined by wal k ( x ) = ( − 1) κ 0 ξ 1 + κ 1 ξ 2 + ··· , where x = (0 .ξ 1 ξ 2 . . . ) 2 ∈ [0 , 1), unique in the sense that infinitely many ξ i are equal to 0. Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 30 / 61
Walsh functions For k = ( . . . κ 1 κ 0 ) 2 ∈ N 0 , the k -th Walsh function is defined by wal k ( x ) = ( − 1) κ 0 ξ 1 + κ 1 ξ 2 + ··· , where x = (0 .ξ 1 ξ 2 . . . ) 2 ∈ [0 , 1), unique in the sense that infinitely many ξ i are equal to 0. For s ≥ 1 and k = ( k 1 , . . . , k s ) ∈ N s 0 , the k -th Walsh function is defined by s ∏ wal k ( x ) = wal k j ( x j ) . j =1 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 30 / 61
Walsh functions Figure: The k -th Walsh functions for k = 0 , 1 , 2 , 3 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 31 / 61
Walsh functions Figure: The k -th Walsh functions for k = 4 , 5 , 6 , 7 Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 32 / 61
Walsh functions Every Walsh function is a piecewise constant function. Takashi Goda (U. Tokyo) Extrapolation and HOQMC MCM 2019 33 / 61
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