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<!-- Weave of 'Summing Primes' generated by Inweb -->
< ul class = "crumbs" > < li > < a href = "../index.html" > Home< / a > < / li > < li > < a href = "index.html" > The Goldbach Conjecture< / a > < / li > < li > < b > Summing Primes< / b > < / li > < / ul > < p class = "purpose" > Here we verify the conjecture for small numbers.< / p >
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< p class = "inwebparagraph" > < a id = "SP1" > < / a > < b > § 1. < / b > So, this is a program to see if even numbers from 4 to 100 can all be written
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as a sum of two primes. Christian Goldbach asked Euler in 1742 if every even
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number greater than 2 can be written this way. This remains open, though —
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< / p >
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< ul class = "items" > < li > (a) every even number is a sum of at most six primes (Ramaré, 1995), and
< / li > < li > (b) every odd number is a sum of at most five (Tao, 2012).
< / li > < / ul >
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< center > < img src = "Letter.jpg" height = "720" > < / center >
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< p class = "inwebparagraph" > Computer verification has been made up to around \(10^{18}\), but by rather better
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methods. We will proceed only to:
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< / p >
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< pre class = "definitions" >
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< span class = "definition-keyword" > define< / span > < span class = "constant-syntax" > RANGE< / span > < span class = "plain-syntax" > < / span > < span class = "constant-syntax" > 100< / span >
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< / pre >
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< pre class = "displayed-code all-displayed-code" >
< span class = "plain-syntax" > #< / span > < span class = "identifier-syntax" > include< / span > < span class = "plain-syntax" > < < / span > < span class = "identifier-syntax" > stdio< / span > < span class = "plain-syntax" > .< / span > < span class = "identifier-syntax" > h< / span > < span class = "plain-syntax" > > < / span >
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< span class = "reserved-syntax" > int< / span > < span class = "plain-syntax" > < / span > < span class = "function-syntax" > main< / span > < span class = "plain-syntax" > (< / span > < span class = "reserved-syntax" > int< / span > < span class = "plain-syntax" > < / span > < span class = "identifier-syntax" > argc< / span > < span class = "plain-syntax" > , < / span > < span class = "reserved-syntax" > char< / span > < span class = "plain-syntax" > *< / span > < span class = "identifier-syntax" > argv< / span > < span class = "plain-syntax" > []) {< / span >
< span class = "plain-syntax" > < / span > < span class = "reserved-syntax" > for< / span > < span class = "plain-syntax" > (< / span > < span class = "reserved-syntax" > int< / span > < span class = "plain-syntax" > < / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > =4; < / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > < < / span > < span class = "constant-syntax" > RANGE< / span > < span class = "plain-syntax" > ; < / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > =< / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > +2) < / span > < span class = "comment" > stepping in twos to stay even< / span >
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< span class = "plain-syntax" > < / span > < < span class = "named-paragraph" > Solve Goldbach's conjecture for i< / span > < span class = "named-paragraph-number" > 1.1< / span > > < span class = "character-syntax" > ;< / span >
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< span class = "character-syntax" > }< / span >
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< / pre > < p class = "inwebparagraph" > < a id = "SP1_1" > < / a > < b > § 1.1. < / b > This ought to print:
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< / p >
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< pre class = "ConsoleText-displayed-code all-displayed-code" >
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< span class = "ConsoleText-plain-syntax" > < / span > < span class = "ConsoleText-element-syntax" > $< / span > < span class = "ConsoleText-plain-syntax" > < / span > < span class = "ConsoleText-function-syntax" > goldbach/Tangled/goldbach< / span >
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< span class = "ConsoleText-plain-syntax" > 4 = 2+2< / span >
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< span class = "ConsoleText-plain-syntax" > 6 = 3+3< / span >
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< span class = "ConsoleText-plain-syntax" > 8 = 3+5< / span >
< span class = "ConsoleText-plain-syntax" > 10 = 3+7 = 5+5< / span >
< span class = "ConsoleText-plain-syntax" > 12 = 5+7< / span >
< span class = "ConsoleText-plain-syntax" > 14 = 3+11 = 7+7< / span >
< span class = "ConsoleText-plain-syntax" > ...< / span >
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< / pre >
< p class = "inwebparagraph" > We'll print each different pair of primes adding up to i. We
only check in the range \(2 \leq j \leq i/2\) to avoid counting pairs
twice over (thus \(8 = 3+5 = 5+3\), but that's hardly two different ways).
< / p >
< p class = "macrodefinition" > < code class = "display" >
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< < span class = "named-paragraph-defn" > Solve Goldbach's conjecture for i< / span > < span class = "named-paragraph-number" > 1.1< / span > > =
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< / code > < / p >
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< pre class = "displayed-code all-displayed-code" >
< span class = "plain-syntax" > < / span > < span class = "identifier-syntax" > printf< / span > < span class = "plain-syntax" > (< / span > < span class = "string-syntax" > "%d"< / span > < span class = "plain-syntax" > , < / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > );< / span >
< span class = "plain-syntax" > < / span > < span class = "reserved-syntax" > for< / span > < span class = "plain-syntax" > (< / span > < span class = "reserved-syntax" > int< / span > < span class = "plain-syntax" > < / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > =2; < / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > < =< / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > /2; < / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > ++)< / span >
< span class = "plain-syntax" > < / span > < span class = "reserved-syntax" > if< / span > < span class = "plain-syntax" > ((< / span > < a href = "S-tsoe.html#SP2" class = "function-link" > < span class = "function-syntax" > isprime< / span > < / a > < span class = "plain-syntax" > (< / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > )) & & (< / span > < a href = "S-tsoe.html#SP2" class = "function-link" > < span class = "function-syntax" > isprime< / span > < / a > < span class = "plain-syntax" > (< / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > -< / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > )))< / span >
< span class = "plain-syntax" > < / span > < span class = "identifier-syntax" > printf< / span > < span class = "plain-syntax" > (< / span > < span class = "string-syntax" > " = %d+%d"< / span > < span class = "plain-syntax" > , < / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > , < / span > < span class = "identifier-syntax" > i< / span > < span class = "plain-syntax" > -< / span > < span class = "identifier-syntax" > j< / span > < span class = "plain-syntax" > );< / span >
< span class = "plain-syntax" > < / span > < span class = "identifier-syntax" > printf< / span > < span class = "plain-syntax" > (< / span > < span class = "string-syntax" > "\n"< / span > < span class = "plain-syntax" > );< / span >
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< / pre > < ul class = "endnotetexts" > < li > This code is used in < a href = "S-sp.html#SP1" > § 1< / a > .< / li > < / ul > < hr class = "tocbar" >
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< ul class = "toc" > < li > < i > (This section begins Sections.)< / i > < / li > < li > < a href = "S-tsoe.html" > Continue with 'The Sieve of Eratosthenes'< / a > < / li > < / ul > < hr class = "tocbar" >
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