1,1

Also 4, together with numbers n such that sum(d|n,(-1)^d) = -A048272(n) = -3 - Benoit Cloitre (benoit7848c(AT)orange.fr), Apr 14 2002

Also, all solutions to the equation sigma(x)+phi(x)=2x+1. - Farideh Firoozbakht, Feb 02 2005

Unique numbers having 3 divisors (1, their square root, themselves). - Alexandre Wajnberg (alexandre.wajnberg(AT)skynet.be), Jan 15 2006

Smallest (or first) new number deleted at the n-th step in an Eratosthenes sieve. - Lekraj Beedassy (blekraj(AT)yahoo.com), Aug 17 2006

Subsequence of semiprimes A001358. - Lekraj Beedassy (blekraj(AT)yahoo.com), Sep 06 2006

A000005(a(n)^(k-1)) = A005408(k) for all k>0. - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Mar 04 2007

Integers having only 1 factor other than 1 and the number itself. Every number in the sequence is a multiple of 1 factor other than 1 and the number itself. 4 : 2 is the only factor other than 1 and 4; 9 : 3 is the only factor other than 1 and 9; and so on. - Rachit Agrawal (rachit_agrawal(AT)daiict.ac.in), Oct 23 2007

The n-th number with p divisors is equal to the n-th prime raised to power p-1, where p is prime. - Omar E. Pol (info(AT)polprimos.com), May 06 2008

There are 2 Abelian groups of order p^2 (C_p^2 and C_p x C_p) and no non-Abelian group. [From Franz Vrabec (franz.vrabec(AT)aon.at), Sep 11 2008]

Free divisors of n or two perfect partitions of n. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Nov 19 2009]

N. J. A. Sloane, Table of n, a(n) for n = 1..5000

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

n such that A062799(n)=2 - Benoit Cloitre (benoit7848c(AT)orange.fr), Apr 06 2002

a(n)=A000040(n)^(3-1)=A000040(n)^2, where 3 is the number of divisors of a(n). - Omar E. Pol (info(AT)polprimos.com), May 06 2008

A000005(a(n))=3 or A002033(a(n))=2. - Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 10 2009

Table[p=Prime[n]; p^2, {n, 1, 30}] [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Aug 07 2008]

(SAGE) BB = primes_first_n(36) list = [] for i in range(36): list.append(BB[i]^2) list - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), May 15 2007

Cf. A000040, A049001, A024450.

Cf. A008864, A060800.

Cf. A000005, A002033. - Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 10 2009

Sequence in context: A082180 A068999 A077438 this_sequence A052043 A030146 A158146

Adjacent sequences: A001245 A001246 A001247 this_sequence A001249 A001250 A001251

nonn,new

N. J. A. Sloane (njas(AT)research.att.com).