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# Sierpiński's triangle

(Redirected from Sierpinski's triangle)

Sierpiński's triangle, named after Wacław Sierpiński, is Pascal's triangle modulo 2.

Sierpiński's triangle (Pascal's triangle mod
 2
)

 n
${\displaystyle {\begin{array}{l}\displaystyle {[{\rm {row}}(n)]_{2}=\left[\sum _{i=0}^{n}{\bigg (}{\binom {n}{i}}~{\bmod {~}}2{\bigg )}~2^{i}\right]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }{\rm {F}}_{i}^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }(2^{2^{i}}+1)^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}}\end{array}}}$
 [row(n)]10
0 1 1
1 1 1 3
2 1 0 1 5
3 1 1 1 1 15
4 1 0 0 0 1 17
5 1 1 0 0 1 1 51
6 1 0 1 0 1 0 1 85
7 1 1 1 1 1 1 1 1 255
8 1 0 0 0 0 0 0 0 1 257
9 1 1 0 0 0 0 0 0 1 1 771
10 1 0 1 0 0 0 0 0 1 0 1 1285
11 1 1 1 1 0 0 0 0 1 1 1 1 3855
12 1 0 0 0 1 0 0 0 1 0 0 0 1 4369
13 1 1 0 0 1 1 0 0 1 1 0 0 1 1 13107
14 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 21845
15 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 65535
16 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 65537
17 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 196611
18 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 327685
19 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 983055
20 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1114129
21 1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 3342387
22 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 5570645
23 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 16711935
24 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 16843009
25 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 50529027
26 1 0 1 0 0 0 0 0 1 0 1 0 0 0 0 0 1 0 1 0 0 0 0 0 1 0 1 84215045
27 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 252645135
28 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 286331153
29 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 858993459
30 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1431655765
31 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4294967295
32 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 4294967297
 i =
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

## Sierpiński's triangle rows

Sierpiński's triangle rows
 n
 [Row(n)]2
 [Row(n)]10
Factorization into Fermat numbers
0 1 1
1 11 3
 (11)2 = 3 = F0
2 101 5
 (101)2 = 5 = F1
3 1111 15
 (11)2 ⋅ (101)2 = 3 ⋅ 5 = F0 F1
4 10001 17
 (10001)2 = 17 = F2
5 110011 51
 (11)2 ⋅ (10001)2 = 3 ⋅ 17 = F0 F2
6 1010101 85
 (101)2 ⋅ (10001)2 = 5 ⋅ 17 = F1 F2
7 11111111 255
 (11)2 ⋅ (101)2 ⋅ (10001)2 = 3 ⋅ 5 ⋅ 17 = F0 F1 F2
8 100000001 257
 (100000001)2 = 257 = F3
9 1100000011 771
 (11)2 ⋅ (100000001)2 = 3 ⋅ 257 = F0 F3
10 10100000101 1285
 (101)2 ⋅ (100000001)2 = 5 ⋅ 257 = F1 F3
11 111100001111 3855
 (11)2 ⋅ (101)2 ⋅ (100000001)2 = 3 ⋅ 5 ⋅ 257 = F0 F1 F3
12 1000100010001 4369
 (10001)2 ⋅ (100000001)2 = 17 ⋅ 257 = F2 F3
13 11001100110011 13107
 (11)2 ⋅ (10001)2 ⋅ (100000001)2 = 3 ⋅ 17 ⋅ 257 = F0 F2 F3
14 101010101010101 21845
 (101)2 ⋅ (10001)2 ⋅ (100000001)2 = 5 ⋅ 17 ⋅ 257 = F1 F2 F3
15 1111111111111111 65535
 (11)2 ⋅ (101)2 ⋅ (10001)2 ⋅ (100000001)2 = 3 ⋅ 5 ⋅ 17 ⋅ 257 = F0 F1 F2 F3
16 10000000000000001 65537
 (10000000000000001)2 = 65537 = F4
17 110000000000000011 196611
 (11)2 ⋅ (10000000000000001)2 = 3 ⋅ 65537 = F0 F4
18 1010000000000000101 327685
 (101)2 ⋅ (10000000000000001)2 = 5 ⋅ 65537 = F1 F4
19 11110000000000001111 983055
 (11)2 ⋅ (101)2 ⋅ (10000000000000001)2 = 3 ⋅ 5 ⋅ 65537 = F0 F1 F4
20 100010000000000010001 1114129
 (10001)2 ⋅ (10000000000000001)2 = 17 ⋅ 65537 = F2 F4
21 1100110000000000110011 3342387
 (11)2 ⋅ (10001)2 ⋅ (10000000000000001)2 = 3 ⋅ 17 ⋅ 65537 = F0 F2 F4
22 10101010000000001010101 5570645
 (101)2 ⋅ (10001)2 ⋅ (10000000000000001)2 = 5 ⋅ 17 ⋅ 65537 = F1 F2 F_4
23 111111110000000011111111 16711935
 (11)2 ⋅ (101)2 ⋅ (10001)2 ⋅ (10000000000000001)2 = 3 ⋅ 5 ⋅ 17 ⋅ 65537 = F0 F1 F2 F4
24 1000000010000000100000001 16843009
 (100000001)2 = (10000000000000001)2 = 257 ⋅ 65537 = F3 F4
25 11000000110000001100000011 50529027
 (11)2 ⋅ (100000001)2 = (10000000000000001)2 = 3 ⋅ 257 ⋅ 65537 = F0 F3 F4
26 101000001010000010100000101 84215045
 (101)2 ⋅ (100000001)2 = (10000000000000001)2 = 5 ⋅ 257 ⋅ 65537 = F1 F3 F4
27 1111000011110000111100001111 252645135
 (11)2 ⋅ (101)2 ⋅ (100000001)2 = (10000000000000001)2 = 3 ⋅ 5 ⋅ 257 ⋅ 65537 = F0 F1 F3 F4
28 10001000100010001000100010001 286331153
 (10001)2 ⋅ (100000001)2 = (10000000000000001)2 = 17 ⋅ 257 ⋅ 65537 = F2 F3 F4
29 110011001100110011001100110011 858993459
 (11)2 ⋅ (10001)2 ⋅ (100000001)2 = (10000000000000001)2 = 3 ⋅ 17 ⋅ 257 ⋅ 65537 = F0 F2 F3 F4
30 1010101010101010101010101010101 1431655765
 (101)2 ⋅ (10001)2 ⋅ (100000001)2 = (10000000000000001)2 = 5 ⋅ 17 ⋅ 257 ⋅ 65537 = F1 F2 F3 F4
31 11111111111111111111111111111111 4294967295
 (11)2 ⋅ (101)2 ⋅ (10001)2 ⋅ (100000001)2 = (10000000000000001)2 = 3 ⋅ 5 ⋅ 17 ⋅ 257 ⋅ 65537 = F0 F1 F2 F3 F4
32 100000000000000000000000000000001 4294967297*
 (100000000000000000000000000000001)2 = 4294967297* = F5
*
 F5 = 4294967297 = 641 ⋅ 6700417
is the first composite Fermat number. From here on no other Fermat number is known to be prime.

### Sierpinski's triangle rows and constructible (with straightedge and compass) odd-sided polygons

For
 n = 1
to
 31
, we get all the non-empty products of the 5 known Fermat primes, giving the number of sides of constructible odd-sided polygons. If there are no other Fermat primes, there are then no more constructible (with straightedge and compass) odd-sided polygons.

### Rows formulae

It can be proved by induction that the rows of Sierpinski's triangle interpreted as a binary number are products of distinct Fermat numbers (Denton Hewgill's identity[1])

${\displaystyle [{\rm {row}}(n)]_{2}=\left[\sum _{i=0}^{n}{\bigg (}{\binom {n}{i}}~{\bmod {~}}2{\bigg )}~2^{i}\right]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }{\rm {F}}_{i}^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }(2^{2^{i}}+1)^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}\,}$

Inductive proof:

• Base case: For row 0, we get 1 (empty product), the product of distinct Fermat numbers in { };
• Inductive hypothesis: Assume that any row  n, 0   ≤   n   ≤   2 k  −  1,
is the product of distinct Fermat numbers in  {F0, ..., Fk  − 1}
${\displaystyle [{\rm {row}}(n)]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }{\rm {F}}_{i}^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }(2^{2^{i}}+1)^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}\,}$
then for any row
 2 k + n, 0   ≤   n   ≤   2 k  −  1,
we have
${\displaystyle [{\rm {row}}(2^{k}+n)]_{2}=[{\rm {row}}(n)]_{2}~[{\rm {F}}_{k}]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(n)\rfloor }{\rm {F}}_{i}^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}~[{\rm {F}}_{k}]_{2}=\left[\prod _{i=0}^{k}{\rm {F}}_{i}^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}=\left[\prod _{i=0}^{\lfloor \log _{2}(2^{k}+n)\rfloor }{\rm {F}}_{i}^{(\lfloor {\frac {n}{2^{i}}}\rfloor ~{\bmod {~}}2)}\right]_{2}.\,}$
As a particular case, rows
 2 n, n   ≥   0,
give
 Fn
, the
 n
th Fermat number.

## Sequences

A006943 Sierpinski's triangle (Pascal's triangle mod 2) rows.

 {1, 11, 101, 1111, 10001, 110011, 1010101, 11111111, 100000001, 1100000011, 10100000101, 111100001111, 1000100010001, 11001100110011, 101010101010101, 1111111111111111, 10000000000000001, 110000000000000011, 1010000000000000101, ...}