OFFSET
1,2
COMMENTS
20 is the first number that is not the sum of substrings of any positive integer. There are 203 such numbers < 10000, and they disproportionately begin with 2 and 3 -- 123 of them and 70 of them, respectively.
LINKS
Christian N. K. Anderson, Table of n, a(n) for n = 1..10000
Christian N. K. Anderson, Table of n, a(n), and all integer solutions.
Christian N. K. Anderson, Ulam Spiral of n = 1..10000, color-coded by their value in this sequence.
EXAMPLE
For a(59)=5, the five solutions are:
136 (because 13+36+1+3+6=59),
140 (because 14+40+1+4+0=59),
317 (because 31+17+3+1+7=59),
321 (because 32+21+3+2+1=59), and
502 (because 50+02+5+0+2=59).
PROG
(R) table(factor(sapply(1:900, function(n) { tot=0; s=as.character(n); len=nchar(s); for(i in 1:len) for(j in i:len) tot=tot+as.numeric(substr(s, i, j)); tot-n } ), levels=1:100))
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Christian N. K. Anderson and Kevin L. Schwartz, May 11 2013
STATUS
approved