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 p. Improved initial writing: 2006-02-27last updated: 2006-02-28

The performance of the monad p. on finding roots has long been known to be not the best. For example, see the article by Nakano, Yamashita, and Nishikawa, in Vector 21.2, Spring 2005; or Kip Murray’s J forum msg from 2001-07-16 with subject “repeated roots”. The problem lies in two main areas:
• limit errors even on small-degree polynomials
• inaccuracies on repeated roots
p. has been improved in both of these areas, as the following examples demonstrate:

```   NB. repeated real roots

p. <2,5\$1
2 _11 25 _30 20 _7 1
p. p. <2,5\$1
+-+-----------+
|1|2 1 1 1 1 1|
+-+-----------+
>{: p. p. <2,5\$1
2 1 1 1 1 1
>{: p. p. <2,10\$1
2 1 1 1 1 1 1 1 1 1 1
>{: p. p. <5 4 # 1 2
2 2 2 2 1 1 1 1 1
> {: p. p. <1r2,8\$1r3
1r2 1r3 1r3 1r3 1r3 1r3 1r3 1r3 1r3

p. <3j4 3j_4
25 _6 1
ptimes=: +//.@(*/)  NB. polynomial multiplication
25 _6 1 ptimes p. <1r2,8\$1r3
_25r13122 328r6561 _7657r13122 26113r6561 _38291r2187 37660r729 _24983r243 ...
> {: p. 25 _6 1 ptimes p. <1r2,8\$1r3
3j4 3j_4 0.5 0.333333 0.333333 0.333333 0.333333 0.333333 0.333333 0.333333 0.333333

NB. Examples from Nakano, Yamashita, and Nishikawa, Vector 21.2, Spring 2005

c=: ''
c=: c,< _4 6 _3 1
c=: c,< _9 9 _3 1
c=: c,< _2 6 _6 1
c=: c,< _18 18 _6 1
c=: c,< _3 9 _9 1
c=: c,< _12 18 _9 1
c=: c,< _1 6 _12 1
c=: c,< _4 12 _12 1
c=: c,< _9 18 _12 1
c=: c,< _5 15 _15 1
c=: c,< _20 30 _15 1
c=: c,< 12 _30 27 _10 1
c=: c,< 2 _3 _3 6 0 _3 1
c=: c,< 175 210 99 _40 _33 _10 1 0 1

r=: >@{:@p.&.> c
_5 ]\ c >./@:|@:p.&> r
0 8.88178e_16 4.44089e_16 3.97205e_15 8.88178e_15
1.77636e_15 6.70575e_14 5.32907e_14  2.4869e_14 5.50671e_14
5.32907e_14 6.92779e_14           0 3.71543e_13           0

NB. the Wilkinson monster

] c=: p. <1+i.20
2432902008176640000 _8752948036761600000 13803759753640704000 _12870931245150988800 ...
> {: p. c
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
(20-i.20) =!.0 > {: p. c
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

NB. Lest you think that all problems have been solved ...

d=: (- 210+2^_23x) _2}c
c =!.0 d
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1
0j20 ": ,.(_2{c),_2{d
_210.00000000000000000000
_210.00000011920928955078

r=: > {: p. d
_5]\ r
20.8469  19.5024j1.94034 19.5024j_1.94034  16.7308j2.81265 16.7308j_2.81265
13.9924j2.51888 13.9924j_2.51888  11.7938j1.65261 11.7938j_1.65261 10.0952j0.644694
10.0952j_0.644694          8.91681          8.00738          6.99968          6.00001
5                4                3                2                1
_5]\ | d p. r
1.04249e13 7.99439e12 7.99439e12 4.73027e11 4.73027e11
2.09392e10 2.09392e10 1.27872e10 1.27872e10  2.86188e9
2.86188e9    3.906e7  8.60529e7  3.70852e7  9.29178e6
274432     565248      13312       4096        512
```

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