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A340799
a(n) is the smallest prime p such that p + 1 has 2n divisors.
3
2, 5, 11, 23, 47, 59, 191, 167, 179, 239, 5119, 359, 20479, 2111, 719, 839, 1114111, 1259, 786431, 3023, 2879, 15359, 348127231, 3359, 22031, 266239, 6299, 6719, 22280142847, 5039, 559419490303, 7559, 156671, 7798783, 25919, 10079, 1168231104511, 5505023
OFFSET
1,1
FORMULA
A000005(a(n) + 1) = 2n.
EXAMPLE
a(4) = 23 because 23 is the smallest prime p such that p + 1 has 2*4 divisors; tau(24) = 8.
PROG
(Magma) Ax:=func<n|exists(r){m:m in[1..10^7] | IsPrime(m) and #Divisors(m + 1) eq n*#Divisors(m)}select r else 0>; [Ax(n): n in[1..20]]
CROSSREFS
Cf. A000005 (tau), A003680, A080371.
Sequence in context: A349411 A347309 A174162 * A186253 A226462 A376193
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Jan 21 2021
STATUS
approved