A211660 - OEIS

A211660

Numbers k such that k and k+2 both have unique values of sigma(k) and sigma(k+2); sigma(k) = A000203(k) = sum of divisors of k.

%I #10 Feb 11 2020 03:54:23

%S 1,2,3,5,7,27,43,98,146,169,171,197,200,217,241,257,281,331,347,379,

%T 386,409,448,461,487,505,507,509,547,554,576,577,641,800,821,829,841,

%U 857,907,937,1117,1250,1261,1283,1289,1322,1352,1359,1387,1415,1601,1621

%N Numbers k such that k and k+2 both have unique values of sigma(k) and sigma(k+2); sigma(k) = A000203(k) = sum of divisors of k.

%C Subsequence of A211656. Number k is in sequence iff k and k+2 are in A211656.

%C Supersequence of A211767 (lesser of twin primes p, p+2 with unique values of sigma(p) and sigma(p+2)).

%H Amiram Eldar, <a href="/A211660/b211660.txt">Table of n, a(n) for n = 1..10000</a>

%e Number 27 is in sequence because sigma(27) = 40, sigma(29) = 30 and there are no other numbers m, n with sigma(m) = 40 or sigma(n) = 30.

%Y Cf. A000203, A211656, A211657, A211658, A211659, A211767, A211768, A211769.

%K nonn

%O 1,2

%A _Jaroslav Krizek_, Apr 20 2012