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 A051152 (-1)sigma sociable number of order 2: (-1)sigma((-1)sigma(x))=x, but (-1)sigma(x)<>x, where if x=Product p(i)^r(i) then (-1)sigma(x)=Product (-1+Sum p(i)^s(i), s(i)=1 to r(i)); (-1)sigma(1)=1. 1
 4, 5, 216, 494, 16800, 21228, 246400, 440220 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Factorizations: 4, 5, 2^3*3^3, 2*13*19, 2^5*3*5^2*7, 2^2*3*29*61 LINKS PROG (PARI) A049060(n)={ local(i, resul, rmax, p) ; if(n==1, return(1) ) ; i=factor(n) ; rmax=matsize(i)[1] ; resul=1 ; for(r=1, rmax, p=0 ; for(j=1, i[r, 2], p += i[r, 1]^j ; ) ; resul *= p-1 ; ) ; return(resul) ; } isA051152(r)={ local(s, t) ; s=A049060(r) ; t=A049060(s) ; if( r == t && s !=r, return(1), return(0) ) ; } { for(n=1, 30000000, if( isA051152(n), print(n, " ") ) ; ) ; } - R. J. Mathar, Oct 12 2006 CROSSREFS Cf. A049057, A049058, A049059, A049060, A034094, A051153. Sequence in context: A331578 A041745 A298224 * A042181 A042717 A304286 Adjacent sequences:  A051149 A051150 A051151 * A051153 A051154 A051155 KEYWORD nonn AUTHOR EXTENSIONS More terms from R. J. Mathar, Oct 12 2006 STATUS approved

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Last modified June 6 04:15 EDT 2020. Contains 334859 sequences. (Running on oeis4.)