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@...>

>

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

(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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