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A307826 The number of integers r such that all primes above a certain value have the form primorial(n)*q +- r. 1
1, 1, 4, 24, 240, 2880, 46080, 829440, 18247680, 510935040, 15328051200, 551809843200, 22072393728000, 927040536576000, 42643864682496000, 2217480963489792000, 128613895882407936000, 7716833752944476160000, 509311027694335426560000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

William Boyles, Table of n, a(n) for n = 1..350

FORMULA

a(n) = Product_{k=1..n} A156037(k).

For n > 1, a(n) = A000010(A002110(n))/2.

EXAMPLE

For n=3, the third primorial is 2*3*5=30, and all primes at least 17 have the form 30n +- (1,7,11,13). So, a(3) = 4.

MATHEMATICA

a[1]=1; a[n_] := EulerPhi[Product[Prime[i], {i, 1, n}]]/2; Array[a, 20] (* Amiram Eldar, Jul 08 2019 *)

PROG

(Python 3)

import sympy

def A307826(n):

    sympy.sieve.extend_to_no(n)

    s = list(sympy.sieve._list)

    prod = s[0]

    print("1")

    for i in range(1, n):

        prod*=s[i]

        print(sympy.ntheory.factor_.totient(prod)//2)

CROSSREFS

Cf. A000010, A002110, A156037.

Sequence in context: A244760 A240403 A074598 * A239840 A052718 A061640

Adjacent sequences:  A307823 A307824 A307825 * A307827 A307828 A307829

KEYWORD

nonn

AUTHOR

William Boyles, Apr 30 2019

STATUS

approved

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Last modified February 27 03:50 EST 2020. Contains 332299 sequences. (Running on oeis4.)