An optimality degree computation unit computes an optimality degree in the case where a first variable included in a variable set is a candidate for an addition variable, using an objective function. An addition threshold computation unit computes an addition threshold based on the computed optimality degree, the addition threshold being a threshold of the optimality degree and indicating a criterion for determining whether or not the first variable is to be set as the candidate for the addition variable. An objective function value computation unit computes an objective function value which is a difference between a value of the objective function computed using variables to be optimized and a value of the objective function computed using the variables to be optimized from which a second variable included in a nonzero variable set is excluded.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. An interactive variable selection device for selecting a variable interactively with a user, the interactive variable selection device comprising: an addition variable candidate computation unit for computing candidates for an addition variable which is a variable to be added from a variable set to a nonzero variable set; an optimal variable selection update unit for updating variables to be optimized, by minimizing, for variables included in the nonzero variable set, an objective function that takes a minimum value in optimized variables; a deletion variable candidate computation unit for computing candidates for a deletion variable which is a variable to be deleted from the nonzero variable set; and a variable selection result output unit for outputting at least one of: the candidates for the addition variable; and the candidates for the deletion variable, wherein the addition variable candidate computation unit includes: an optimality degree computation unit for computing an optimality degree in the case where a first variable included in the variable set is a candidate for the addition variable, using the objective function; and an addition threshold computation unit for computing an addition threshold based on the computed optimality degree, the addition threshold being a threshold of the optimality degree and indicating a criterion for determining whether or not the first variable is to be set as the candidate for the addition variable, and wherein the deletion variable candidate computation unit includes: an objective function value computation unit for computing an objective function value which is a difference between a value of the objective function computed using the variables to be optimized and a value of the objective function computed using the variables to be optimized from which a second variable included in the nonzero variable set is excluded; and a deletion threshold computation unit for computing a deletion threshold based on the computed objective function value, the deletion threshold being a threshold of the objective function value and indicating a criterion for determining whether or not the second variable is to be deleted from the nonzero variable set. 2. The interactive variable selection device according to claim 1, wherein the optimality degree computation unit solves a one-variable optimization problem of minimizing the objective function for the first variable, to compute a value of the objective function as the optimality degree. 3. The interactive variable selection device according to claim 1, wherein the optimality degree computation unit computes, for the first variable, a gradient of the objective function in the variables to be optimized, as the optimality degree. 4. The interactive variable selection device according to claim 1, comprising a replacement variable candidate computation unit for receiving a variable that is included in the nonzero variable set and is designated to be replaced with a variable included in the variable set, and computing, from the variable set, candidates for the variable that is to replace the designated variable. 5. The interactive variable selection device according to claim 2, comprising a replacement variable candidate computation unit for receiving a variable that is included in the nonzero variable set and is designated to be replaced with a variable included in the variable set, and computing, from the variable set, candidates for the variable that is to replace the designated variable. 6. The interactive variable selection device according to claim 3, comprising a replacement variable candidate computation unit for receiving a variable that is included in the nonzero variable set and is designated to be replaced with a variable included in the variable set, and computing, from the variable set, candidates for the variable that is to replace the designated variable. 7. The interactive variable selection device according to claim 4, wherein the replacement variable candidate computation unit includes: an optimal variable primary update unit for optimizing the objective function using a set of variables in which the designated variable is excluded from the candidates for the addition variable; and a replacement threshold computation unit for computing a replacement threshold based on the computed optimality degree, the replacement threshold being a threshold indicating a criterion for determining whether or not the designated variable is to be replaced with the variable included in the variable set, and wherein the optimality degree computation unit computes the optimality degree in the case where the first variable included in the variable set including the designated variable is the candidate for the addition variable, using the objective function. 8. The interactive variable selection device according to claim 5, wherein the replacement variable candidate computation unit includes: an optimal variable primary update unit for optimizing the objective function using a set of variables in which the designated variable is excluded from the candidates for the addition variable; and a replacement threshold computation unit for computing a replacement threshold based on the computed optimality degree, the replacement threshold being a threshold indicating a criterion for determining whether or not the designated variable is to be replaced with the variable included in the variable set, and wherein the optimality degree computation unit computes the optimality degree in the case where the first variable included in the variable set including the designated variable is the candidate for the addition variable, using the objective function. 9. The interactive variable selection device according to claim 6, wherein the replacement variable candidate computation unit includes: an optimal variable primary update unit for optimizing the objective function using a set of variables in which the designated variable is excluded from the candidates for the addition variable; and a replacement threshold computation unit for computing a replacement threshold based on the computed optimality degree, the replacement threshold being a threshold indicating a criterion for determining whether or not the designated variable is to be replaced with the variable included in the variable set, and wherein the optimality degree computation unit computes the optimality degree in the case where the first variable included in the variable set including the designated variable is the candidate for the addition variable, using the objective function. 10. The interactive variable selection device according to claim 1, wherein the variable selection result output unit outputs a variable determined to be the candidate for the addition variable based on the addition threshold, or a variable determined to be deleted from the candidates for the addition variable based on the deletion threshold. 11. The interactive variable selection device according to claim 1, wherein the addition threshold computation unit specifies optimality degrees in order of decreasing optimality degree, and computes, as the addition threshold, a value obtained by multiplying a mean value of the specified optimality degrees by a parameter that is not less than 0 and not more than 1, the optimality degrees being as many as a number obtained by subtracting the number of variables added to the nonzero variable set from the number of nonzero elements assumed to remain in the variables to be optimized. 12. An interactive variable selection method for selecting a variable interactively with a user, the interactive variable selection method comprising: computing candidates for an addition variable which is a variable to be added from a variable set to a nonzero variable set; updating variables to be optimized, by minimizing, for variables included in the nonzero variable set, an objective function that takes a minimum value in optimized variables; computing candidates for a deletion variable which is a variable to be deleted from the nonzero variable set; and outputting at least one of: the candidates for the addition variable; and the candidates for the deletion variable, wherein when computing the candidates for the addition variable, an optimality degree in the case where a first variable included in the variable set is a candidate for the addition variable is computed using the objective function, and an addition threshold is computed based on the computed optimality degree, the addition threshold being a threshold of the optimality degree and indicating a criterion for determining whether or not the first variable is to be set as the candidate for the addition variable, and wherein when computing the candidates for the deletion variable, an objective function value which is a difference between a value of the objective function computed using the variables to be optimized and a value of the objective function computed using the variables to be optimized from which a second variable included in the nonzero variable set is excluded is computed, and a deletion threshold is computed based on the computed objective function value, the deletion threshold being a threshold of the objective function value and indicating a criterion for determining whether or not the second variable is to be deleted from the nonzero variable set. 13. The interactive variable selection method according to claim 12, wherein when computing the optimality degree, a one-variable optimization problem of minimizing the objective function for the first variable is solved to compute a value of the objective function as the optimality degree. 14. A non-transitory computer readable information recording medium storing an interactive variable selection program applied to a computer for selecting a variable interactively with a user, when executed by a processor, the interactive variable selection program performs a method for: computing candidates for an addition variable which is a variable to be added from a variable set to a nonzero variable set; updating variables to be optimized, by minimizing, for variables included in the nonzero variable set, an objective function that takes a minimum value in optimized variables; computing candidates for a deletion variable which is a variable to be deleted from the nonzero variable set; and outputting at least one of: the candidates for the addition variable; and the candidates for the deletion variable, wherein when computing the candidates for the addition variable, an optimality degree in the case where a first variable included in the variable set is a candidate for the addition variable is computed using the objective function, and an addition threshold is computed based on the computed optimality degree, the addition threshold being a threshold of the optimality degree and indicating a criterion for determining whether or not the first variable is to be set as the candidate for the addition variable, and wherein when computing the candidates for the deletion variable, an objective function value which is a difference between a value of the objective function computed using the variables to be optimized and a value of the objective function computed using the variables to be optimized from which a second variable included in the nonzero variable set is excluded is computed, and a deletion threshold is computed based on the computed objective function value, the deletion threshold being a threshold of the objective function value and indicating a criterion for determining whether or not the second variable is to be deleted from the nonzero variable set. 15. The non-transitory computer readable information recording medium according to claim 14, wherein when computing the optimality degree, a one-variable optimization problem of minimizing the objective function for the first variable is solved to compute a value of the objective function as the optimality degree.