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 A216587 Preimage of n for A216556 (i.e., concatenation of digits of a(n), each increased by 1, yields n), -1 if there is none. 5
 -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 10, 11, 12, 13, 14, 15, 16, 17, 18, -1, 20, 21, 22, 23, 24, 25, 26, 27, 28, -1, 30, 31, 32, 33, 34, 35, 36, 37, 38, -1, 40, 41, 42, 43, 44, 45, 46, 47, 48, -1, 50, 51, 52, 53, 54, 55, 56, 57, 58, -1, 60, 61, 62, 63 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Also: Left inverse to A216556: A216587 o A216556 = id. Sequence A216589 lists the indices n for which a(n)=-1, i.e., n is not in the range of A216556. LINKS E. Angelini, Strings resurrection, SeqFan mailing list, Sep 08 2012 FORMULA a(A216556(n))=n for all n. a(n)=n-1 for n < 11; a(n)=n-11 for 20 < n < 110 except n=30, 40, ..., 100. a(n)=-1 if n contains a substring "20", "30",..., "90" or "00", or if n starts with digits "11", "12", ..., "19". For all other n>1 one has a(n)>0. EXAMPLE a(a(a(2127)))=a(a(1016))=a(905)=-1, since under A216556, 905 -> 1016 -> 2127, but no n yields A216556(n)=905. PROG (PARI) A216587(n)={my(s=0); n!=1&for(i=1, #n=Vecsmall(Str(n)), n[i]>48||(i>1&n[i-1]==49&s=s\10+1)||return(-1); (s=s*10+n[i]-49)&next; (i<#n&n[i++]==48&s=9)||return(-1)); s} CROSSREFS See also A216557. Sequence in context: A000030 A179635 A306354 * A174210 A134777 A253015 Adjacent sequences:  A216584 A216585 A216586 * A216588 A216589 A216590 KEYWORD sign,base,easy AUTHOR M. F. Hasler, Sep 09 2012 STATUS approved

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Last modified October 22 04:25 EDT 2019. Contains 328315 sequences. (Running on oeis4.)