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A213653 Least semiprime whose digital sum is n, or 0 if no such integer exists. 2
0, 10, 10001, 21, 4, 14, 6, 25, 26, 9, 46, 38, 39, 49, 77, 69, 169, 278, 0, 289, 299, 489, 589, 689, 699, 799, 899, 0, 2899, 3899, 4989, 5899, 5999, 6999, 7999, 9899, 0, 19999, 29999, 48999, 58999, 68999, 69999, 88999, 99899, 0, 299899, 398999, 589989 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

a(9k) = 0 for all k>1.

I conjecture that all terms > 278, except for 10001, end in the digit "9". What is the next term a(n) > 69 violating monotony, i.e., such that a(n) < a(n-1)? M. F. Hasler, Jun 17 2012

a(88) = 7999999999 < a(87) = 8899899999. - Alois P. Heinz, Jun 17 2012

a(76) = 499999999 < a(75) = 597999999. - Donovan Johnson, Jun 18 2012

LINKS

Donovan Johnson and Alois P. Heinz, Table of n, a(n) for n = 0..400 (first 101 terms from Donovan Johnson)

MATHEMATICA

semiPrimeQ[n_] := PrimeOmega[n] == 2; t = Table[0, {100}]; k = 1; While[k < 10^7, If[ semiPrimeQ@ k, s = Plus @@ IntegerDigits@ k; If[s < 101 && t[[s]] == 0, t[[s]] = k; Print[{s, k}]]]; k++]

PROG

(PARI) A213653(n)={ n%9 || n==9 || return; forstep( a=A051885(n), 9e9, 9, bigomega(a)==2 || next; A007953(a)==n & return(a))} \\ - M. F. Hasler, Jun 17 2012

CROSSREFS

Sequence in context: A163019 A013746 A076782 * A283889 A283890 A139109

Adjacent sequences:  A213650 A213651 A213652 * A213654 A213655 A213656

KEYWORD

easy,base,nonn

AUTHOR

Robert G. Wilson v, Jun 17 2012

EXTENSIONS

a(0)-a(62) double-checked with given PARI code by M. F. Hasler, Jun 17 2012

STATUS

approved

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Last modified January 23 04:16 EST 2020. Contains 331168 sequences. (Running on oeis4.)