A263877 -id:A263877 - cp's OEIS frontend

A263876 Numbers n such that n^2 + 1 has two distinct prime divisors less than n.

7, 18, 38, 41, 68, 70, 182, 239, 500, 682, 776, 800, 1068, 1710, 1744, 4030, 4060, 5604, 5744, 8119, 12156, 15006, 16610, 17684, 21490, 25294, 26884, 27590, 32060, 32150, 37416, 37520, 45630, 47321, 58724, 71264, 84906, 88526, 98864, 109054, 109610, 128766

Offset: 1

Michel Lagneau, Oct 28 2015

nonn

Subsequence of A256011.

The numbers n such that n^2 + 1 = p*q are semiprimes (A085722) are not in the sequence. According to this property, the corresponding sequence of the number of prime divisors with multiplicity is 3, 3, 3, 3, 4, 3, 5, 5, 3, 5, 3, 3, 7, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 6, ...

			7 is in the sequence because 7^2 + 1 = 2*5^2 => 2 and 5 are less than 7.

Amiram Eldar, Table of n, a(n) for n = 1..500

Cf. A085722, A144255, A256011, A263877.

Select[Range[150000], PrimeNu[#^2+1] == 2&&FactorInteger[#^2+1][[1,1]]<# &&FactorInteger[#^2+1][[2,1]]<#&]

for(n=1, 1e5, t=n^2+1; if ((omega(t) == 2) && (factor(t)[, 1][2] < n), print1(n, ", "))); \\ Altug Alkan, Oct 28 2015

cp's OEIS Frontend

A263876 Numbers n such that n^2 + 1 has two distinct prime divisors less than n.

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