Find squares n where n+1 is prime: Difference between revisions

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1 4 16 36 100 196 256 400 576 676

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;;; Find squares n where n + 1 is prime

PROGRAM primesq

INCLUDE library

;;; returns TRUE(1) if p is prime, FALSE(0) otherwise

FUNC BYTE isPrime

ARG WORD n

WORD i

WORD f

WORD f2

WORD toNext

BYTE prime

BEGIN

IF n < 3

prime = n = 2

ELSE IF n % 3 = 0

prime = n = 3

ELSE IF n % 2 = 0

prime = 0

ELSE

prime = 1

f = 5

f2 = 25

toNext = 24 ; note: ( 2n + 1 )^2 - ( 2n - 1 )^2 = 8n

WHILE f2 <= n AND prime

prime = n % f <> 0

f = f + 2

f2 = toNext

toNext = toNext + 8

RETURN prime

END

WORD i2

WORD toNext

BEGIN

IF isPrime( ( 1 * 1 ) + 1 ) ; 1 is the only possible odd number

OUTPUT " 1"

i2 = 4

toNext = 4 ; note: ( 2n + 2 )^2 - 2n^2 = 8n + 4

WHILE i2 < 1000

IF isPrime( i2 + 1 )

OUTPUT " #W", i2

toNext = toNext + 8

i2 = i2 + toNext

END