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A307546 Prime indices of A195685. 1
7, 14, 15, 20, 26, 28, 29, 36, 45, 48, 66, 70, 89, 98, 104, 115, 122, 126, 142, 152, 157, 161, 164, 167, 177, 182, 186, 191, 194, 199, 202, 205, 216, 218, 219, 244, 264, 279, 283, 295, 297, 299, 324, 329, 336, 342, 362, 408, 416, 423, 430, 440, 457, 498, 500 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Since only the indices are listed, this sequence is more compact than A195685. As described there, all elements of the triples (2*prime(a(n))-1, 2*prime(a(n)), 2*prime(a(n))+1) are products of exactly two distinct primes. Such values are called "squarefree semiprimes" in A006881.

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A000720(A195685(n)).

A195685(n) = A000040(a(n)).

EXAMPLE

A000040(a(1)) = 17 = A195685(1).

A000040(a(5)) = A000040(26) = 101 = A195685(5).

MAPLE

with(numtheory):

q:= n-> (p-> tau(2*p-1)=4 and tau(2*p+1)=4)(ithprime(n)):

select(q, [$1..500])[];  # Alois P. Heinz, Apr 18 2019

CROSSREFS

Cf. A000005, A000040, A000720, A006881, A195685.

Sequence in context: A232797 A107976 A022557 * A297261 A296705 A297138

Adjacent sequences:  A307543 A307544 A307545 * A307547 A307548 A307549

KEYWORD

nonn

AUTHOR

Peter Dolland, Apr 14 2019

STATUS

approved

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Last modified May 19 18:35 EDT 2022. Contains 353847 sequences. (Running on oeis4.)