- Here is a little contribution about handwavey methods. It does not

specifically deal with Carmichaels, although my first examinations

are coming up with some interesting material on them as well.

Let us consider f(a,b) = b^2-a^2-a*b and f(c,d) = d^2-c^2-c*d

a,b,c,d all integers. Actually it works for all a,b,c,d.

Take

a,c

b,d

a+b,c+d

Multiply each line and get

a*c (L1)

b*d (L2)

a*c+b*c+b*c+b*d (L3)

Take (L1)+(L2) (L4)

and (L2)+(L3) (L5)

Take (L5)-(L4) (L6)

You will find that (L6)^2 - (L4)*(L5) = +/-f(a,b)*f(b,c)

Do the same for

a,-d

b,c

a+b,c-d

and you will get a new L6,L5,L4 which gives +/-f(a,b)*f(b,c)

Thus you have two new functions which give the product ot the first 2

A worked example

31 = 7^2-2*9 and 11 = 4^2-1*5

2*1=2

7*4=28

9*5=45

2+28=30

28+45=73

73-30=43

43^2-30*73=-341=-31*11

2*(-4)=-8

7*1=7

9*(-3)=-27

-8+7=-1

7-27=-20

-20-(-1)=-19

(-19)^2 -(-1)(-19) = 341

This method will work for all Lucas sequences, with appropriate

changes in the original polynomial coefficients.

John McNamara - Apologies for the aborted attempt at a post. 'Tab' followed by either

'space' or 'return' (too quick to know what happened) seems to be a

lethal key combination.

From: "mcnamara_gio" <mcnamara_gio@...>>

2, 3, and 5 can never be factors. This boosts density by a factor of

> What do you think about this sequence a(n)=n^2+7n-1. Its terms are

> usually prime. I have calculated that 72% of a(n) so that n<500 is

> prime. 81% of a(n) is prime when n<5000. 85.6% of a(n) is prime when

> n<50000 and 88.5% of a(n) is prime when n<500000. I am going to find

> more prime terms in this sequence. What do you think about it?

(2/1)*(3/2)*(5/4) over arbitrary ranges. However, 7,11, 13 and 17 both

divide 2 of the p possible residues. This decreases density by a factor

of (5/6)*(9/10)*(11/12)*(15/16) over arbitrary ranges.

Looking at primes up to 10000, the density boost is almost exactly 2.75.

This is pretty feeble compared with Euler's famous trinomials.

Run this script in Pari/GP:

rnorm=1.0

rthis=1.0

forprime(p=2,10000,roots=polrootsmod(x^2+7*x-1,p)~;rnorm*=(p-1)/p;rthis*=(p-#roots)/p;print(p"

"rthis" "rnorm" "roots))

print(rthis/rnorm);

Research the Euler trinomials, and try the above script on them too, to see why

I say 2.75 is pretty feeble.

Phil

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