Calculate Pi unto a point using the Nilakantha series

Question

Your task: given a nonzero positive number i, calculate pi using the Nilakantha series unto i terms.

The Nilakantha series is as follows:

$$\text 3 + \frac{4}{2 \times 3 \times 4} - \frac{4}{4 \times 5\times 6}+\frac{4}{6 \times 7 \times 8} - ...$$

3 is the first term, 4/2*3*4 is the second, -4/4*5*6 is the third, and so on.

Notice that for the nth term:

• $$\text S_1 = 3$$
• $$\text S_n = \frac{4 \times (-1)^n}{2n \times (2n-1) \times (2n-2)}$$
• The approximation of pi by summing up these terms is $$\text S_1 +\text S_2\text + … +\text S_n$$

Test cases:

In = Out

1 = 3
2 = 3.16666666667
3 = 3.13333333333
4 = 3.1452381

Notice the pattern of the numbers approximating towards pi.

Floating point issues are OK.

This is code-golf so shortest answer wins!

EDIT: by default this is 1-indexed, but if you want 0-indexed no problem, just mention it. And even infinitely printing the approximations with no input is Okay.

EDIT 2: Why some activity here?

Answer 1

Python 3.8 (pre-release), 38 bytes (@xnor)

f=lambda n,i=.5:i//n/n or 2/i-f(n,i+1)

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Python 3.8 (pre-release), 40 bytes (@xnor)

f=lambda n,i=1:i//2//n/n or 4/i-f(n,i+2)

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Python, 45 bytes

f=lambda n,s=1:4/(2*s-1)-(s//n/s or f(n,s+1))

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1-based. n has to be positive.

Python, 50 bytes

f=lambda n,s=0:n and f(n-1,4/(n+n-1)-s-0**s/n)or s

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This uses \$\frac 4 {2x(2x+1)(2x+2)} =\frac 1 x + \frac 1 {x+1} - \frac 4 {2x+1} \$ and that inside the full sum the first two terms cancel.

1-based. Can handle 0.

Python 3.8 (pre-release), 45 bytes (@xnor)

f=lambda n:0**n*3or(-1)**n/n/(n-~n)/~n+f(n-1)

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Python, 50 bytes

f=lambda n:0**n*3or 1/(n|1)/(~n-n)/(n%-2^n)+f(n-1)

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0-based.

Answer 2

Jelly, 13 bytes

RḤrƝP€4÷Ṛḅ-+3

A monadic Link that accepts a positive integer and yields the approximation (up to the floating point accuracy).

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How?

RḤrƝP€4÷Ṛḅ-+3 - Link: positive integer, n  e.g. 4
R             - range                           [1,2,3,4]
 Ḥ            - double                          [2,4,6,8]
   Ɲ          - for neighbours:
  r           -   inclusive range               [[2,3,4],[4,5,6],[6,7,8]]
    P€        - product of each                 [24,120,336]
      4÷      - four divided by those           [1/6,1/30,1/84]
        Ṛ     - reverse                         [1/84,1/30,1/6]
         ḅ-   - convert from base -1            sum([1/84,-1/30,1/6])=0.14523809523809522
           +3 - add three                       3.14523809523809522

Answer 3

JavaScript (ES6), 32 bytes

This version is based on @loopy-walt's answer, golfed by @xnor.

f=(n,i=.5)=>i<n?2/i-f(n,i+1):1/n

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---

JavaScript (ES6), 39 bytes

f=n=>--n?f(n)+(4-n%2*8)/(n+=n)/++n/~n:3

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Commented

f = n =>          // f is a recursive function taking the input n
--n ?             // decrement n; if it's not 0:
  f(n) +          //   do a recursive call and add:
  (4 - n % 2 * 8) //     -4 if n is odd, +4 otherwise
  / (n += n)      //     divided by 2n
  / ++n           //     divided by 2n + 1
  / ~n            //     divided by -(2n + 2)
:                 // else:
  3               //   end of recursion: return the integer part

Answer 4

Factor + koszul math.unicode,  ⁶⁸ 64 bytes

[ ""3 rot [0,b) [ -1^ 4 reach Π / * + [ 2 v+n ] dip ] each ]

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0-indexed. Note the string literal has the control characters 2, 3, and 4 embedded, making it equivalent to the sequence { 2 3 4 }. You can see these characters on ATO.

              ! 3
""            ! 3 { 2 3 4 }
3             ! 3 { 2 3 4 } 3
rot           ! { 2 3 4 } 3 3
[0,b)         ! { 2 3 4 } 3 { 0 1 2 }
[ ... ] each     <<for each element in { 0 1 2 }...>>
                 <<first iteration>>
              ! { 2 3 4 } 3 0
-1^           ! { 2 3 4 } 3 1
4             ! { 2 3 4 } 3 1 4
reach         ! { 2 3 4 } 3 1 4 { 2 3 4 }
Π             ! { 2 3 4 } 3 1 4 24
/             ! { 2 3 4 } 3 1 1/6
*             ! { 2 3 4 } 3 1/6
+             ! { 2 3 4 } 3+1/6
[ 2 v+n ] dip ! { 4 5 6 } 3+1/6
                 <<second iteration>>
              ! { 4 5 6 } 3+1/6 1
                 <<and so on>>

Answer 5

Vyxal ḋ, 19 bytes

ƛune4*nd:‹:‹**/;∑3+

ḋ flag to print rationals in their decimal form

Explanation:

ƛune4*nd:‹:‹**/;∑3+
ƛ              ;      Map lambda through inclusive range 1 to input
 une                  Push -1 to the power n
    4*                Multiply by 4 and push that
      nd:             Multiply n by 2 and duplicate
         ‹:           Decrement and duplicate
           ‹**        Decrement and push product of denominator
              /       Divide 4*(-1)**n by 2n*(2n-1)*(2n-2)
                ∑3+   Sum list and add 3

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Answer 6

R, 51 bytes

\(k,n=2:k*2)`if`(k>1,3+sum(4*1i^n/n/(n-1)/(n-2)),3)

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Uses the fact that \$(-1)^n=i^{2n}\$.

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R, 51 bytes

\(k,n=2:k)`if`(k>1,3+sum((-1)^n/n/(2*n-1)/(n-1)),3)

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Uses the formula but with simplifying the fraction to \$\frac{(-1)^n}{n(2n-1)(n-1)}\$.

Answer 7

05AB1E, 20 16 bytes

3λè®Nm4*N·2Ý-P/+

Outputs the 1-based \$n^{th}\$ value (by starting with \$a(0)=3\$ and where \$a(1)\$ is calculated as \$3\$ as well).

Try it online or verify the infinite sequence.

Explanation:

 λ         # Start a recursive environment,
  è        # to calculate a(input)
           # (which is output implicitly afterwards)
3          # Start with a(0)=3
           # Where every following a(n) is calculated by:
           #  (implicitly push the previous term a(n-1))
   ®Nm     #  Push (-1) to the power n
      4*   #  Multiply it by 4
   N·      #  Push 2n
     2Ý    #  Push list [0,1,2]
       -   #  Subtract each from the 2n: [2n,2n-1,2n-2]
        P  #  Take the product of this triplet: 2n*(2n-1)*(2n-2)
   /       #  Divide the earlier 4*(-1)**n by this (2n*(2n-1)*(2n-2))
    +      #  Add it to the previous term a(n-1)

Answer 8

><>, 36 bytes

Prints the infinite sequence

23:nao$:1+$:2%8*4$-$2*:1-:1-**,{+10.

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><>, 41 bytes

Generates the nth term

3$v ;n~<
1->:2(?v::2%8*4$-$2*:1-:1-**,{+}

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Answer 9

C (gcc), ^{80 \$\cdots\$ 71} 70 bytes

i;float s,m;float f(n){for(m=4,s=i=3;i++<2*n;s-=m/i/~-i/(i++-2))m=-m;}

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Saved 5 6 bytes thanks to Kevin Cruijssen!!!
Saved 4 bytes thanks to Arnauld!!!

Answer 10

Desmos, 36 bytes

f(x)=3+∑_{n=2}^x(-1)^n/n(2nn-3n+1)

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Near direct copy of the definition, simplified and rearranged only a little bit. 1 indexed.

Breakdown:

f(x)=3+∑_{n=2}^x(-1)^n/n(2nn-3n+1)  full function

f(x)=                               function definition (not sure if required)
     3+                             3 plus
       ∑                            The sum from
        _{n=2}                        n=2
              ^x                      to n=x
                                      (note that this defaults to 0 if x is less than 2)
                                    of
                (-1)^n                -1 when n is odd, 1 if n is even
                      /               divided by
                       n(2nn-3n+1)    2n^3-3n^2+n

Answer 11

Vyxal ḋr, 14 bytes

Ḣƛd2ʀεΠ4/;uβ3+

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Ḣ              # range(2, n)
 ƛ       ;     # map...
     ε         # Difference of...
  d            # 2x
   2ʀ          # [0, 1, 2] 
      Π        # Take the product
       4/      # 4 / that
          uβ   # Convert from base -1 (alternate signs)
            3+ # Add 3

Answer 12

Python 3, 49 bytes

f=lambda n:n-1and(-1)**n/(2*n*n-n)/~-n+f(n-1)or 3

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-5 bytes thanks to Mukundan314

Answer 13

Wolfram Language (Mathematica), 36 35 bytes

3.-Sum[(-1)^n/(2n^3+3n^2+n),{n,#}]&

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Alternatively, and more interestingly, we can express the partial sums in terms of a Lerch transcendent: 36 bytes

Pi+(-1)^#(1/#-2LerchPhi[-1,1,#+.5])&

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Answer 14

x86 32-bit machine code, 37 33 bytes

D9 E8 D8 C0 D8 C0 D9 E8 F9 D9 E8 D8 F1 D9 E8 DE C2 D8 F1 D8 C0 F5 72 F5 DE E2 E2 ED DD D8 D9 E1 C3

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Following the fastcall calling convention, this takes a number i in ECX and returns the sum of the first i terms on the FPU register stack.

In assembly:

f:  fld1
    fadd st(0), st(0)
    fadd st(0), st(0)   # Example execution for 2nd iteration
    fld1                # FPU register stack (left is bottom):
    stc                 # -3   2   (before)
r:  fld1                # -3   2   1
    fdiv st(0), st(1)   # -3   2   1/2
ir: fld1                # -3   2   1/2   1
    faddp st(2), st(0)  # -3   3   1/2
    fdiv st(0), st(1)   # -3   3   1/(2*3)
    fadd st(0), st(0)   # -3   3   2/(2*3)
    cmc                 #[Repeat the 4 instructions above, using an inner loop:
    jc ir               # -3   3   2/(2*3)   1
                        # -3   4   2/(2*3)
                        # -3   4   2/(2*3*4)
                        # -3   4   4/(2*3*4)
    fsubrp st(2), st(0) # 3+4/(2*3*4)   4
    loop r
    fstp st(0)
    fabs
    ret

Each iteration of the outer loop computes one term \$\frac4{2n(2n+1)(2n+2)}\$ and combines it in using the reverse-subtract instruction; as in my answer to a related problem, this handles the alternating signs, but leaves the result with the wrong sign for even i, which is corrected for by taking the absolute value at the end (because all the correct results are positive).

The first iteration, if left the same as later iterations, would contain a division by 0. This is prevented by setting CF to 1 at the beginning, so that the inner loop executes once instead of twice during the first iteration, and it computes \$\frac2{1\cdot2}\$ instead.

Answer 15

Raku, 36 bytes

[\+] 3,{-4*($*=-1)/[*] ^3+2*++$}...*

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This is an expression for the infinite sequence of partial sums. 3 is the first term of the sequence, and the curly braces enclose a generating expression for the subsequent terms.

• The dividend is -4 * ($ *= -1). The $ here is an anonymous state variable. The *= -1 causes it to alternate between -1 and 1. (The first time the expression is evaluated, it's undefined, but since it's being multiplied, it defaults to the multiplicative identity element 1.) Multiplying that by -4 produces the sequence of dividends 4, -4, 4, -4, ....
• The divisor is [*] ^3 + 2 * ++$. $ here is another anonymous state variable which the preincrement operator ++ causes to take on the values 1, 2, 3, ..., as it's evaluated for each term of the sequence. Multiplying that by 2 produces 2, 4, 6, .... Those even numbers are added to the range ^3, which means the integers from 0 to 2, producing a sequence of ranges 2, 3, 4, 4, 5, 6, 6, 7, 8, .... Then [*] multiplies those numbers together.

[\+] produces the sequence of partial sums of the terms.

Answer 16

Fig, \$18\log_{256}(96)\approx\$ 14.816 bytes

+3S\n2@Nere+r3hax4

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Port of Vyxal. Beats both that and osabie, 1.8 bytes longer than Jelly. 0-indexed.

+3S\n2@Nere+r3hax4
               ax  # Range [1, n]
              h    # Double
            r3     # [0, 1, 2]
          e+       # Add ^ to every element of ^^
        er         # Product of each element
      @N           # Negate
    n2             # Every other element
   \             4 # Four divided by ^
  S                # Sum
+3                 # Add 3

Answer 17

Charcoal, 22 21 bytes

Try it online! Link is to verbose version of code. Explanation:

   Ｎ                     Input `n` as a number
  Ｅ                      Map over implicit range
     ι                   Current value
    ⎇                    If nonzero then
          ι              Current value
         ﹪               Modulo
           ²             Literal integer `2`
        ⊗                Doubled
       ⊖                 Decremented
      ∕                  Divided by
              ι         Current value
             ×          Multiplied by
                ι       Current value
               ⊕        Incremented
            ×           Multiplied by
                   ι    Current value
                  ⊗     Doubled
                 ⊕      Incremented
                     ³   Otherwise literal integer `3`
 Σ                       Take the sum
Ｉ                        Cast to string
                       Implicitly print

Edit: Saved 1 byte by adpating @pajonk's simplification.