2.1. The Influence of Public Capital on Enterprise Technological Innovation
According to the different functions of public capital, it can be divided into economic public capital and social public capital. The economic public capital, such as water and electricity, transportation and storage, environmental facilities, information software, and other fields, can improve enterprise profits and stimulate technological innovation motivation to directly promote enterprise technological innovation by reducing transportation costs, production costs, and market transaction costs. At the same time, economic public capital indirectly promotes enterprise technological innovation through its positive externalities to form industrial agglomeration. Economic public capital can provide basic conditions and investment opportunities for enterprise production and operation, guide enterprises to transfer to public capital-intensive areas, and form industrial agglomeration to affect enterprise technological innovation [
18,
19]. The social public capital includes education, science and technology, health and social security, culture and sports, public management, and other fields. Among them, public education capital can promote enterprise technological innovation by improving the quantity and quality of human capital [
20]. Public R&D capital promotes enterprise technological innovation mainly in the following three aspects: increasing R&D investment through pre-funding and post-subsidy, forming an R&D resource-sharing mechanism through the establishment of R&D public service platform, solving the problems of resource sharing and information asymmetry, and improving innovation efficiency through industry–university–research cooperation model [
21,
22]. The increase of public capital investment by the government in medical care, culture and sports, public management, and social organizations can ensure the physical and mental health of enterprise employees and reduce the transaction costs between enterprises by forming a culture of mutual trust and promoting organizational communication and contact. Therefore, social public capital can directly or indirectly promote enterprise technological innovation [
23].
Hypothesis 1 (H1). Public capital promotes enterprise technological innovation directly or indirectly.
2.2. The Nonlinear Regulating Effect of Industrial Agglomeration on the Innovation Effect of Public Capital
According to the external economic theory [
24], economic agglomeration refers to the interaction between enterprises, institutions, and infrastructure in a certain spatial region in the production process, forming economies of scale and scope, promoting the development of the labor market, and concentrating on specialized skills, thus increasing the interaction between regional suppliers and consumers and sharing infrastructure and other regional externalities. In theory, public capital can promote the formation of industrial agglomeration in the area of public capital investment by creating a public resource endowment, reducing production costs, and accelerating talent agglomeration [
25]. First of all, from the perspective of creating a public resource endowment, the public capital is the public infrastructure formed by the government relying on national rights to concentrate financial resources in a certain area. Enterprises in the investment area can obtain government public goods and services at a lower cost, form the cost advantage of public services, and induce industrial agglomeration. Secondly, from the perspective of reducing production costs, the public capital is conducive to reducing the comprehensive transportation costs, search costs, transaction costs, and energy consumption costs; reducing the resistance to industrial agglomeration; improving the matching rate and success rate of the market transactions; increasing the marginal income of enterprise technological innovation; forming an innovation network; and promoting industrial agglomeration [
26,
27]. Finally, from the perspective of accelerating the concentration of human capital, the public capital investment in education, medical care, social security, and other infrastructure forms the “siphon effect” of human capital, accelerates the development of human capital, and makes the value of the human capital increase in the movement, thus promoting industrial agglomeration [
28].
However, the influence of public capital on enterprise technological innovation may change in different stages of industrial agglomeration [
29].
From the objective conditions and laws of forming industrial agglomeration, asset-light, low-cost, and small-scale enterprises can take the lead in forming industrial agglomeration in the primary stage in the public capital investment area due to their short decision-making time and fast transfer speed. Most of these enterprises are at the low end of the value chain, with weak independent innovation ability; insufficient technology absorption ability; low requirements for production technology, capital scale, and human capital; and insufficient reserves of technical talents and innovation talents, which can hardly meet the needs of enterprise technological innovation and development. At the same time, in the stage of primary industrial agglomeration, the scientific and technological chain and industrial chain between enterprises are not effectively connected. The transformation of scientific and technological achievements is insufficient. A specialized division of labor and cooperation is not yet formed. The degree of trust among enterprises is very low, and there is little cooperation. Therefore, in the initial stage of industrial agglomeration, enterprises also intensify the degree of market competition among them while sharing public resources. The competition further reduces the profit level of enterprises, leads to the outflow of innovative talents, reduces the R&D input of enterprises, and weakens the promoting effect of public capital investment on enterprise technological innovation. In short, in the initial stage of industrial agglomeration, the higher the degree of industrial agglomeration, the weaker the impact of public capital investment on enterprise technological innovation.
The process of accelerating the formation of industrial agglomeration by the public capital is also the process of transforming industrial agglomeration from the primary stage to the advanced stage [
30]. On the one hand, through the competition mechanism of the survival of the fittest in the market, excellent enterprises with abundant funds and strong innovation ability survive in the industrial agglomeration area, merge some enterprises, and form industrial giants. On the other hand, with time, large enterprises with heavy assets, high costs, and strong innovation capabilities are gradually gathered in the areas of public capital and finally realize the transformation of industrial agglomeration from the primary stage to the advanced stage [
31]. In the advanced stage of industrial agglomeration, the cooperative relationship among enterprises is greater than the competition, and the talent agglomeration effect of public capital begins to appear [
32]. At this time, enterprises in the region have strong innovation consciousness and technology absorption ability and are good at integrating market resources. The collaborative innovation mechanism among enterprises is thus formed [
33]. Leading enterprises and small- and medium-sized enterprises jointly builds a collaborative innovation network [
34], jointly integrates scientific and technological resources, shares innovation achievements, carries out technical cooperation, and jointly overcomes technical problems. Scientific and technological links, industrial chains, and capital chains are thus gradually formed, which greatly improves the conversion rate of scientific and technological achievements, further raises the profit level of enterprises, and improves the ability to coordinate and innovate among enterprises. In short, in the advanced stage of industrial agglomeration, the higher the degree of industrial agglomeration, the stronger the influence of public capital investment on enterprise technological innovation. Accordingly, this paper puts forward hypothesis 2.
Hypothesis 2 (H2). In the initial stage of industrial agglomeration, the higher the degree of industrial agglomeration, the weaker the innovation effect of public capital; in the advanced stage of industrial agglomeration, the higher the degree of industrial agglomeration, the stronger the innovation effect of public capital.