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A336093 Let P(n) = primorial(n) = A002110(n); a(n) is the number of primes q < P(n) such that P(n) - q is also prime and q^2==1 (mod P(n)). 0
0, 0, 2, 4, 2, 8, 6, 28, 36, 40, 56, 106, 192, 304, 526, 926, 1644, 2756, 4944, 8840, 15958, 28402, 51102, 92372 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

A totative of k is a number <= k and relatively prime to k.

A336016(n) gives the number of prime totatives p of P(n) for which p^2==1 (mod P(n)). Whereas P(n) - p also has this property, it is not always prime. This sequence gives the number of prime totatives q of P(n) such that q^2==1 mod P(n) and P(n) - q is also prime. All terms are even; a(n) <= A336016(n) for all n. The number of distinct representations of P(n) as the sum of two primes each having multiplicative order 2 (mod P(n)) is given by a(n)/2, for which A116979(n) is an upper bound.

LINKS

Table of n, a(n) for n=1..24.

EXAMPLE

P(4)=210; all totatives 29,41,71,139,181 are prime. However 210 - 41 = 169 is not prime, whereas 210-29 = 181, 210-71 = 139. Therefore the totatives we count in this case are 29,71,139,181, so a(4) = 4.

MAPLE

with(NumberTheory):

P := proc (k)

local n, v, W, H;

n := 1; v := 0;

W := product(ithprime(j), j = 1 .. k);

H := PrimeCounting(W);

for n from 1 to H do

if mod(ithprime(n)^2, W) = 1 and isprime(W-ithprime(n)) then v := v+1 else v := v end if:

end do:

v;

end proc:

seq(P(k), k = 1 .. 8);

MATHEMATICA

{0, 0}~Join~Table[Block[{P = #, k = 0}, Do[If[AllTrue[{#, P - #}, And[PrimeQ@ #, MultiplicativeOrder[#, P] == 2] &], k++] &@ Prime[i], {i, PrimePi[n + 1], PrimePi[P/2]}]; 2 k ] &@ Product[Prime@ j, {j, n}], {n, 3, 8}]

CROSSREFS

Cf. A000010, A336015, A336016, A116979.

Sequence in context: A279350 A278221 A287879 * A304213 A129178 A152874

Adjacent sequences:  A336090 A336091 A336092 * A336094 A336095 A336096

KEYWORD

nonn,more

AUTHOR

David James Sycamore, Michael De Vlieger Jul 09 2020

STATUS

approved

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Last modified May 10 23:31 EDT 2021. Contains 343784 sequences. (Running on oeis4.)